Semiconductor device and driving method thereof

ABSTRACT

A semiconductor device including a nonvolatile memory cell in which a writing transistor which includes an oxide semiconductor, a reading transistor which includes a semiconductor material different from that of the writing transistor, and a capacitor are included is provided. Data is written to the memory cell by turning on the writing transistor and applying a potential to a node where a source electrode (or a drain electrode) of the writing transistor, one electrode of the capacitor, and a gate electrode of the reading transistor are electrically connected, and then turning off the writing transistor, so that the predetermined amount of charge is held in the node. Further, when a p-channel transistor is used as the reading transistor, a reading potential is a positive potential.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/740,688, filed Jun. 16, 2015, now U.S. Pat. No. 9,525,051, which is acontinuation of U.S. application Ser. No. 14/552,551, filed Nov. 25,2014, now U.S. Pat. No. 9,299,813, which is a continuation of U.S.application Ser. No. 13/861,681, filed Apr. 12, 2013, now U.S. Pat. No.8,902,640, which is a continuation of U.S. application Ser. No.13/195,105, filed Aug. 1, 2011, now U.S. Pat. No. 8,422,272, whichclaims the benefit of foreign priority applications filed in Japan asSerial No. 2010-176982 on Aug. 6, 2010, and Serial No. 2011-108051 onMay 13, 2011, all of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The disclosed invention relates to a semiconductor device using asemiconductor element and a method for manufacturing the semiconductordevice. The disclosed invention also relates to a driving method of thesemiconductor device.

2. Description of the Related Art

Storage devices using semiconductor elements are broadly classified intotwo categories: a volatile device that loses stored data when powersupply stops, and a non-volatile device that retains stored data evenwhen power is not supplied.

A typical example of a volatile storage device is a DRAM (dynamic randomaccess memory). A DRAM stores data in such a manner that a transistorincluded in a storage element is selected and electric charge is storedin a capacitor.

On the above-described principle, when data is read from a DRAM, chargein a capacitor is lost; thus, another writing operation is necessaryevery time data is read out. Moreover, since leakage current (off-statecurrent) flows between a source and a drain of a transistor included ina memory element when the transistor is in an off state, charge flowsinto or out even if the transistor is not selected, which makes a dataholding period short. For that reason, another writing operation(refresh operation) is necessary at predetermined intervals, and it isdifficult to sufficiently reduce power consumption. Furthermore, sincestored data is lost when power supply stops, an additional storagedevice using a magnetic material or an optical material is needed inorder to hold the data for a long time.

Another example of a volatile storage device is an SRAM (static randomaccess memory). An SRAM holds stored data by using a circuit such as aflip-flop and thus does not need refresh operation. This means that anSRAM has an advantage over a DRAM. However, cost per storage capacity isincreased because a circuit such as a flip-flop is used. Moreover, as ina DRAM, stored data in an SRAM is lost when power supply stops.

A typical example of a non-volatile storage device is a flash memory. Aflash memory includes a floating gate between a gate electrode and achannel formation region in a transistor and stores data by holdingelectric charge in the floating gate. Therefore, a flash memory hasadvantages in that the data holding time is extremely long (almostpermanent) and refresh operation which is necessary in a volatilestorage device is not needed (e.g., Patent Document 1).

However, a gate insulating layer included in a storage elementdeteriorates by tunneling current generated in writing, so that thestorage element stops its function after a predetermined number ofwriting operations. In order to reduce adverse effects of this problem,a method in which the number of writing operations for storage elementsis equalized is employed, for example. However, a complicated peripheralcircuit is needed to realize this method. Moreover, employing such amethod does not solve the fundamental problem of lifetime. In otherwords, a flash memory is not suitable for applications in which data isfrequently rewritten.

In addition, high voltage is necessary in order to inject charge intothe floating gate or removing the charge, and a circuit therefor isrequired. Further, it takes a relatively long time to inject or removecharge, and it is not easy to increase the speed of writing and erasingdata.

REFERENCE

Patent Document 1: Japanese Published Patent Application No. S57-105889

SUMMARY OF THE INVENTION

In view of the foregoing problems, an object of one embodiment of thedisclosed invention is to provide a semiconductor device having a novelstructure, which can hold stored data even when not powered and whichhas an unlimited number of write cycles.

In the disclosed invention, a semiconductor device is formed using amaterial which allows a sufficient reduction in off-state current of atransistor; for example, an oxide semiconductor material, which is awide-gap semiconductor, is used. When a semiconductor material whichallows a sufficient reduction in off-state current of a transistor isused, the semiconductor device can hold data for a long period.

Further, one embodiment of the disclosed invention provides asemiconductor device including a nonvolatile memory cell including awriting transistor which includes an oxide semiconductor, a readingtransistor which includes a semiconductor material different from thatof the writing transistor, and a capacitor. Data is written or rewrittento the memory cell by turning on the writing transistor and applying apotential to a node where one of a source electrode and drain electrodeof the writing transistor, one electrode of the capacitor, and a gateelectrode of the reading transistor are electrically connected, and thenturning off the writing transistor, so that the predetermined amount ofcharge is held in the node. Further, when a p-channel transistor is usedas the reading transistor, a reading potential is a positive potential.

Specifically, structures described below can be employed, for example.

One embodiment of the present invention is a semiconductor device thatincludes a bit line, a source line, a write word line, a write-read wordline, and a memory cell. The memory cell includes a first transistorwhich is a p-channel transistor and includes a first gate electrode, afirst source electrode, a first drain electrode, and a first channelformation region; a second transistor including a second gate electrode,a second source electrode, a second drain electrode, and a secondchannel formation region; and a capacitor. The first channel formationregion contains a semiconductor material different from that of thesecond channel formation region. The first gate electrode, the seconddrain electrode, and one electrode of the capacitor are electricallyconnected to form a node where charge is held. The bit line, the firstsource electrode, and the second source electrode are electricallyconnected; the source line and the first drain electrode areelectrically connected; the write word line and the second gateelectrode are electrically connected; and the write-read word line andthe other electrode of the capacitor are electrically connected.

Another embodiment of the present invention is a semiconductor devicethat includes a bit line, a source line, a write word line, a write-readword line, a memory cell array including a plurality of memory cells,and a potential switching circuit. One of the memory cells includes afirst transistor which is a p-channel transistor and includes a firstgate electrode, a first source electrode, a first drain electrode, and afirst channel formation region; a second transistor including a secondgate electrode, a second source electrode, a second drain electrode, anda second channel formation region; and a capacitor. The first channelformation region contains a semiconductor material different from thatof the second channel formation region. The first gate electrode, thesecond drain electrode, and one electrode of the capacitor areelectrically connected to form a node where charge is held. The bitline, the first source electrode, and the second source electrode areelectrically connected; one terminal of the potential switching circuit,the source line, and the first drain electrode are electricallyconnected; the write word line and the second gate electrode areelectrically connected; the write-read word line and the other electrodeof the capacitor are electrically connected; and the source line iselectrically connected to the memory cells in a plurality of columns.The potential switching circuit has a function of selectively supplyinga ground potential to the source line in a writing period.

In the above semiconductor device, the second channel formation regionpreferably includes an oxide semiconductor.

In the above semiconductor device, the second transistor is preferablyprovided so as to overlap with at least part of the first transistor.

In the above semiconductor device, the first channel formation regionmay include silicon.

In the above semiconductor device, the second transistor may be ann-channel transistor.

Another embodiment of the present invention is a driving method of asemiconductor device that includes a bit line, a source line, aplurality of write word lines, a plurality of write-read word lines, anda memory cell array including a plurality of memory cells. One of thememory cells includes a first p-channel transistor including a firstgate electrode, a first source electrode, a first drain electrode, and afirst channel formation region; a second transistor including a secondgate electrode, a second source electrode, a second drain electrode, anda second channel formation region; and a capacitor. The first gateelectrode, the second drain electrode, and one electrode of thecapacitor are electrically connected to form a node where charge isheld. The bit line, the first source electrode, and the second sourceelectrode are electrically connected; the source line and the firstdrain electrode are electrically connected; one of the write word linesand the second gate electrode are electrically connected; and one of thewrite-read word lines and the other electrode of the capacitor areelectrically connected. A ground potential is supplied to the sourceline in a writing period. In a reading period, a power supply potentialis supplied to one of the write-read word lines which is connected toone of the memory cells in a non-selected state.

Note that in this specification and the like, the term such as “over” or“below” does not necessarily mean that a component is placed “directlyon” or “directly under” another component. For example, the expression“a gate electrode over a gate insulating layer” can mean the case wherethere is an additional component between the gate insulating layer andthe gate electrode.

In addition, in this specification and the like, the term such as“electrode” or “wiring” does not limit a function of a component. Forexample, an “electrode” is sometimes used as part of a “wiring”, andvice versa. In addition, the term ∀electrode∀ or ∀wiring∀ can also meana combination of a plurality of ∀electrodes∀ and ∀wirings∀, for example.

Functions of a “source” and a “drain” are sometimes replaced with eachother when a transistor of opposite polarity is used or when thedirection of current flowing is changed in circuit operation, forexample. Therefore, the terms “source” and “drain” can be replaced witheach other in this specification.

Note that in this specification and the like, the ten “electricallyconnected” includes the case where components are connected through anobject having any electric function. There is no particular limitationon an object having any electric function as long as electric signalscan be transmitted and received between components that are connectedthrough the object.

Examples of an “object having any electric function” are a switchingelement such as a transistor, a resistor, an inductor, a capacitor, andan element with a variety of functions as well as an electrode and awiring.

Since the off-state current of a transistor including an oxidesemiconductor is extremely low, stored data can be held for an extremelylong time by using the transistor. In other words, power consumption canbe adequately reduced because refresh operation is not needed or thefrequency of refresh operation can be extremely low. Moreover, storeddata can be held for a long time even when power is not supplied (notethat a potential is preferably fixed).

Further, a semiconductor device according to one embodiment of thedisclosed invention does not need high voltage for writing of data andthere is no problem of deterioration of elements. For example, unlike aconventional non-volatile memory, it is not necessary to inject andextract electrons into and from a floating gate, and thus a problem suchas deterioration of a gate insulating layer does not occur at all. Inother words, the semiconductor device according to the disclosedinvention has no limitation on the number of times data can berewritten, which is a problem of a conventional nonvolatile memory, andthe reliability thereof is drastically improved. Furthermore, since datais written by turning on or off the transistor, high-speed operation canbe easily realized. Additionally, there is an advantage in thatoperation for erasing data is not needed.

When a transistor which includes a material other than an oxidesemiconductor and can operate at sufficiently high speed is used as areading transistor in combination with a transistor which includes anoxide semiconductor and is used as a writing transistor, a semiconductordevice can perform operation (e.g., data reading) at sufficiently highspeed. Further, a transistor including a material other than an oxidesemiconductor can favorably realize a variety of circuits (e.g., a logiccircuit or a driver circuit) which needs to be able to operate at highspeed.

A semiconductor device having a novel feature can be realized by beingprovided with both the transistor including a material other than anoxide semiconductor (in other words, a transistor capable of operatingat sufficiently high speed) and the transistor including an oxidesemiconductor (in a broader sense, a transistor whose off-state currentis sufficiently small).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1, 1A-2, and 1B are circuit diagrams of a semiconductor device.

FIGS. 2A and 2B are circuit diagrams of a semiconductor device.

FIG. 3 is a circuit diagram of a semiconductor device.

FIG. 4 is a timing chart.

FIGS. 5A and 5B are a cross-sectional view and a plan view of asemiconductor device.

FIGS. 6A to 6G are cross-sectional views illustrating manufacturingsteps of a semiconductor device.

FIGS. 7A to 7E are cross-sectional views illustrating manufacturingsteps of the semiconductor device.

FIGS. 8A to 8D are cross-sectional views illustrating manufacturingsteps of the semiconductor device.

FIGS. 9A to 9D are cross-sectional views illustrating manufacturingsteps of the semiconductor device.

FIGS. 10A to 10C are cross-sectional views illustrating manufacturingsteps of the semiconductor device.

FIGS. 11A to 11F are views each illustrating an electronic deviceincluding a semiconductor device.

FIGS. 12A to 12E are views each illustrating a crystal structure of anoxide material.

FIGS. 13A to 13C are views illustrating a crystal structure of an oxidematerial.

FIGS. 14A to 14C are views illustrating a crystal structure of an oxidematerial.

FIG. 15 is a graph showing gate voltage dependence of mobility, which isobtained by calculation.

FIGS. 16A to 16C are graphs showing gate voltage dependence of draincurrent and mobility, which is obtained by calculation.

FIGS. 17A to 17C are graphs showing gate voltage dependence of draincurrent and mobility, which is obtained by calculation.

FIGS. 18A to 18C are graphs showing gate voltage dependence of draincurrent and mobility, which is obtained by calculation.

FIGS. 19A and 19B are views each illustrating a cross-sectionalstructure of a transistor used in the calculation.

FIGS. 20A to 20C are graphs each showing characteristics of a transistorincluding an oxide semiconductor film.

FIGS. 21A and 21B are graphs each showing V_(g)−I_(d) characteristicsafter a BT test of a transistor that is Sample 1.

FIGS. 22A and 22B are graphs each showing V_(g)−I_(d) characteristicsafter a BT test of a transistor that is Sample 2.

FIG. 23 is a graph showing V_(g) dependence of I_(d) (a solid line) andfield-effect mobility.

FIGS. 24A and 24B are a graph showing the relationship between substratetemperature and threshold voltage and a graph showing the relationshipbetween substrate temperature and field effect mobility, respectively.

FIG. 25 is a graph showing XRD spectra of Sample A and Sample B.

FIG. 26 is a graph showing the relationship between off-state current ofa transistor and substrate temperature at measurement.

FIGS. 27A and 27B are a top view and a cross-sectional view of acoplanar top-gate top-contact transistor including an In—Sn—Zn—O film asan oxide semiconductor film.

FIGS. 28A and 28B are a top view and a cross-sectional view illustratinga structure of a transistor manufactured in Example 2.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments and examples of the present invention will bedescribed with reference to the drawings. Note that the presentinvention is not limited to the following description and it will bereadily appreciated by those skilled in the art that the modes anddetails of the present invention can be modified in various ways withoutdeparting from the spirit and scope thereof. Therefore, the inventionshould not be construed as being limited to the description in followingthe embodiments and examples.

Note that the position, size, range, and the like of each componentillustrated in the drawings and the like are not accurately representedfor easy understanding in some cases. Therefore, the disclosed inventionis not necessarily limited to the position, size, range, and the like inthe drawings and the like.

In this specification and the like, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

Embodiment 1

In this embodiment, a basic circuit structure and operation of asemiconductor device according to one embodiment of the disclosedinvention will be described with reference to FIGS. 1A-1, 1A-2, and 1Band FIGS. 2A and 2B. Note that in each of circuit diagrams, in somecases, “OS” is written beside a transistor in order to indicate that thetransistor includes an oxide semiconductor.

<Basic Circuit 1>

First, the most basic circuit configuration and its operation will bedescribed with reference to FIGS. 1A-1, 1A-2, and 1B. In a semiconductordevice illustrated in FIG. 1A-1, a bit line BL, a source electrode (or adrain electrode) of a transistor 160, and a source electrode (or a drainelectrode) of a transistor 162 are electrically connected; and a sourceline SL and the drain electrode (or the source electrode) of thetransistor 160 are electrically connected. A write word line OSG and agate electrode of the transistor 162 are electrically connected. A gateelectrode of the transistor 160 and the drain electrode (or the sourceelectrode) of the transistor 162 are electrically connected to oneelectrode of a capacitor 164. A write-read word line C and the otherelectrode of the capacitor 164 are electrically connected. Note that thesource electrode (or the drain electrode) of the transistor 160 and thesource electrode (or the drain electrode) of the transistor 162 may beconnected to different wirings without being electrically connected toeach other.

Here, a transistor including an oxide semiconductor is used as thetransistor 162, for example. A transistor including an oxidesemiconductor has a characteristic of a significantly small off-statecurrent. For that reason, a potential of the gate electrode of thetransistor 160 can be held for an extremely long time by turning off thetransistor 162. When the capacitor 164 is provided, holding of chargesupplied to the gate electrode of the transistor 160 and reading ofstored data can be facilitated.

Note that there is no particular limitation on a semiconductor materialused for the transistor 160. In terms of increasing the speed of readingdata, it is preferable to use, for example, a transistor with highswitching rate such as a transistor formed using single crystal silicon.Note that a p-channel transistor is used as the transistor 160.

The capacitor 164 may be omitted as in FIG. 1B.

The semiconductor device in FIG. 1A-1 can write, hold, and read data asdescribed below, utilizing a characteristic in which the potential ofthe gate electrode of the transistor 160 can be held.

First, writing and holding of data will be described. First, thepotential of the write word line OSG is set to a potential which allowsthe transistor 162 to be turned on, so that the transistor 162 is turnedon. Accordingly, the potential of the bit line BL is supplied to a node(also referred to as a node FG) where the drain electrode (or the sourceelectrode) of the transistor 162, the gate electrode of the transistor160, and one electrode of the capacitor 164 are electrically connected.In other words, predetermined charge is supplied to the node FG (datawriting). Here, one of charges for supply of two different potentials(hereinafter, a charge for supply of a low potential is referred to as acharge Q_(L) and a charge for supply of a high potential is referred toas a charge Q_(H)) is supplied. Note that charges giving three or moredifferent potentials may be supplied to improve a storage capacitor.After that, the potential of the write word line OSG is set to apotential which allows the transistor 162 to be turned off, so that thetransistor 162 is turned off. Thus, the charge supplied to the node FGis held (data holding).

Since the off-state current of the transistor 162 is significantlysmall, the charge of the gate electrode of the transistor 160 is heldfor a long time.

Secondly, reading of data will be described. When an appropriatepotential (a reading potential) is supplied to the write-read word lineC with the source line SL supplied with a predetermined potential (aconstant potential), the bit line BL has a potential which variesdepending on the amount of charge held in the node FG. In other words,the conductance of the transistor 160 is controlled by the charge heldin the gate electrode of the transistor 160 (which can also be referredto as the node FG).

In general, when the transistor 160 is a p-channel transistor, anapparent threshold voltage V_(th) _(_) _(H) in the case where Q_(H) issupplied to the gate electrode of the transistor 160 is lower than anapparent threshold voltage V_(th) _(_) _(L) in the case where Q_(L) issupplied to the gate electrode of the transistor 160. For example, inthe case where Q_(L) is supplied in writing, when the potential of thewrite-read word line C is V₀ (a potential intermediate between V_(th)_(_) _(H) and V_(th) _(_) _(L)), the transistor 160 is turned on. In thecase where Q_(H) is supplied in writing, even when the potential of thewrite-read word line C is V₀, the transistor 160 remains off. Thus, thedata held can be read out by detecting the potential of the bit line BL.

Thirdly, rewriting of data will be described. Rewriting of data isperformed in a manner similar to that of the writing and holding ofdata. In other words, the potential of the write word line OSG is set toa potential which allows the transistor 162 to be turned on, whereby thetransistor 162 is turned on. Accordingly, the potential of the bit lineBL (a potential for new data) is supplied to the node FG. After that,the potential of the write word line OSG is set to a potential whichallows the transistor 162 to be turned off, whereby the transistor 162is turned off. Accordingly, charge for new data is held in the node FG.

In the semiconductor device according to the invention disclosed, datacan be directly rewritten by another writing of data as described above.Therefore, extracting of charge from a floating gate with the use of ahigh voltage needed in a flash memory or the like is not necessary andthus, reduction in operation speed, which is attributed to erasingoperation, can be suppressed. In other words, high-speed operation ofthe semiconductor device can be realized.

Writing, holding, and reading of data, for example, in the case whereeither a potential V_(DD) or a ground potential GND is supplied to thenode FG will be specifically described below. In the description below,data that is held when the potential V_(DD) is supplied to the node FGis referred to as data “1”, and data that is held when the groundpotential GND is supplied to the node FG is referred to as data “0”.Note that the relationship among potentials supplied to the node FG isnot limited to this example.

In the case where data is written, the source line SL is set to GND, thewrite-read word line C is set to GND, and the write word line OSG is setto V_(DD), so that the transistor 162 is turned on. Then, in the casewhere data “0” is written to the node FG, GND is supplied to the bitline BL; in the case where data “1” is written to the node FG, thepotential of the bit line BL is set to V_(DD). Note that in the casewhere data “1” is written to the node FG, the potential of the writeword line OSG may be set to V_(DD)V_(th) _(_) _(OS) in order to preventthe potential supplied to the node FG from being lower than V_(DD) bythe threshold voltage of the transistor 162 (V_(th) _(_) _(OS)).

In the case where the data is held, the write word line OSG is set toGND, so that the transistor 162 is turned off. The bit line BL and thesource line SL are made to have the same potential in order to preventpower consumption due to current flowing in the bit line BL and thesource line SL through the transistor 160 that is a p-channel transistor(hereinafter, also the p-channel transistor 160). Note that thepotential of the write-read word line C may be either V_(DD) or GND aslong as the bit line BL and the source line SL have the same potential.

Note that in the above description, “the same potential” includes“substantially the same potential”. In other words, the purpose of theabove operation lies in that the potential difference between the bitline BL and the source line SL is sufficiently reduced to reduce currentflowing in the bit line BL and the source line SL; therefore, “the samepotential” includes “substantially the same potential”, for example, apotential which allows power consumption to be sufficiently reduced (toone hundredth or less) compared to the case where the potential of thesource line SL is fixed to GND or the like. In addition, potentialdeviation due to wiring resistance or the like are reasonablyacceptable.

In the case where the data is read out, the write word line OSG is setto GND, the write-read word line C is set to GND, and the source line SLis set to V_(DD) or a potential which is slightly lower than V_(DD)(hereinafter, referred to as V_(R)). Here, in the case where data “1”has been written to the node FG, the p-channel transistor 160 is turnedoff, and the potential of the bit line BL is maintained at the level ofthe beginning of reading or is increased. Note that it depends on areading circuit connected to the bit line BL whether the potential ofthe bit line BL is maintained or increased. Further, in the case wheredata “0” has been written to the node FG, the transistor 160 is turnedon, and the potential of the bit line BL becomes V_(DD) or V_(R) whichis equal to the potential of the source line SL. Thus, data “1” or data“0” held in the node FG can be read out by detecting the potential ofthe bit line BL.

Note that in the case where a potential V_(DD) is held in the node FG(that is, data “1” has been written to the node FG), when the potentialof the source line SL is set to V_(DD) at the time of reading, thevoltage between the gate and the source of the transistor 160(hereinafter, referred to as V_(gsp)) becomes 0 V(V_(gsp)=V_(DD)−V_(DD)=0 V), so that V_(gsp) is higher than thethreshold voltage of the transistor 160 (hereinafter, referred to asV_(th) _(_) _(p)); thus, the p-channel transistor 160 is turned off.Here, even in the case where the potential held in the node FG is lowerthan V_(DD) because, for example, the potential written to the node FGis lower than V_(DD), as long as the potential of the node FG is higherthan or equal to V_(DD)−|V_(th) _(_) _(p)|, the equation,V_(gsp)=(V_(DD)−|V_(th) _(_) _(p)|)−V_(DD)=−|V_(th) _(_) _(p)|=V_(th)_(_) _(p), is satisfied and the transistor 160 is turned off; thus, data“1” can be read correctly. However, in the case where the potential ofthe node FG is lower than V_(DD)−|V_(th) _(_) _(p)|, V_(gsp) is lowerthan V_(th) _(_) _(p); thus, the transistor 160 is turned on and notdata “1” but data “0” is read, which is incorrect data reading. In otherwords, in the case where data “1” has been written, the lowest potentialat which data “1” can be read is V_(DD)−|V_(th) _(_) _(p)| which islower than the potential of the source line SL by |V_(th) _(_) _(p)|.

In contrast, when the potential of the source line SL is set to V_(R) atthe time of reading, the lowest potential which allows data “1” to beread out is V_(R)−|V_(th) _(_) _(p)| which is lower than the potentialV_(R) of the source line SL by |V_(th) _(_) _(p)|, as described above.Here, since V_(R) is lower than V_(DD), V_(R)−|V_(th) _(_) _(p)| islower than V_(DD)−|V_(th) _(_) _(p)|. In other words, the lowestpotential which allows data “1” to be read out in the case where thepotential of the source line SL is set to V_(R) is lower than that inthe case where the potential of the source line SL is set to V_(DD).Thus, setting the potential of the source line SL to V_(R) is preferableto setting the potential to V_(DD), in which case the range of thepotential which allows reading of data “1” can be widened. As for thehighest potential which allows data to be read out, in the case wherethe potential of the source line SL is set to V_(R), V_(gsp) in the casewhere V_(DD) has been written to the node FG is V_(DD)−V_(R)>V_(th) _(_)_(p) (because of V_(DD)>V_(R)); thus, the transistor 160 can be turnedoff without any problem.

Here, the node (the node FG) where the drain electrode (or the sourceelectrode) of the transistor 162, the gate electrode of the transistor160, and one electrode of the capacitor 164 are electrically connectedhas a function similar to that of a floating gate of a floating-gatetransistor used for a nonvolatile memory element. When the transistor162 is off, the node FG can be regarded as being embedded in aninsulator and charge is held in the node FG. The off-state current ofthe transistor 162 including an oxide semiconductor is smaller than orequal to one hundred thousandth of the off-state current of a transistorincluding a silicon semiconductor or the like; thus, loss of the chargeaccumulated in the node FG due to leakage current of the transistor 162is negligible. In other words, with the transistor 162 including anoxide semiconductor, a nonvolatile memory device which can store datawithout being supplied with power can be realized.

For example, when the off current of the transistor 162 is 10 zA (1 zA(zeptoampere) is 1×10⁻²¹ A) or less at room temperature (25° C.) and thecapacitance value of the capacitor 164 is approximately 10 fF, data canbe stored for 10⁴ seconds or longer. Needless to say, the holding timedepends on transistor characteristics and the capacitance value.

Further, the semiconductor device according to an embodiment of thedisclosed invention does not have the problem of deterioration of a gateinsulating layer (a tunnel insulating film), which is a problem of aconventional floating-gate transistor. In other words, the problem ofdeterioration of a gate insulating layer due to injection of electronsinto a floating gate, which has been regarded as a problem, can besolved. This means that there is no limit on the number of times ofwriting in principle. Furthermore, a high voltage needed for writing orerasing in a conventional floating-gate transistor is not necessary.

Components such as transistors in the semiconductor device in FIG. 1A-1can be regarded as including resistors and capacitors as illustrated inFIG. 1A-2. In other words, in FIG. 1A-2, the transistor 160 and thecapacitor 164 are each regarded as including a resistor and a capacitor.R1 and C1 denote the resistance and the capacitance of the capacitor164, respectively. The resistance R1 corresponds to the resistance ofthe insulating layer included in the capacitor 164. R2 and C2 denote theresistance and the capacitance of the transistor 160, respectively. Theresistance R2 corresponds to the resistance of the gate insulating layerat the time when the transistor 160 is turned on. The capacitance C2corresponds to a so-called gate capacitance (capacitance formed betweenthe gate electrode and the source or drain electrode, and capacitanceformed between the gate electrode and the channel formation region).

A charge holding period (also referred to as a data holding period) isdetermined mainly by the off-state current of the transistor 162 underthe conditions where the gate leakage current of the transistor 162 issufficiently small and R1 and R2 satisfy R1≧ROS and R2≧ROS, where ROS isthe resistance (also referred to as effective resistance) between thesource electrode and the drain electrode in a state where the transistor162 is turned off.

On the other hand, in the case where the conditions are not satisfied,it is difficult to secure a sufficient holding period even if theoff-state current of the transistor 162 is small enough. This is becausea leakage current other than the off-state current of the transistor 162(e.g., a leakage current generated between the source electrode and thegate electrode) is large. Accordingly, it can be said that it ispreferable that the semiconductor device disclosed in this embodimentsatisfies the above relationships of R1≧ROS and R2≧ROS.

Meanwhile, it is desirable that C1 and C2 satisfy C1≧C2. This is becauseif C1 is large, when the potential of the node FG is controlled by theread-write word line C, the potential of the read-write word line C canbe efficiently supplied to the node FG and the difference betweenpotentials supplied to the read-write word line C (e.g., a readingpotential and a non-reading potential) can be kept small.

As described above, when the above relation is satisfied, a morefavorable semiconductor device can be realized. Note that R1 and R2 arecontrolled by the gate insulating layer of the transistor 160 and theinsulating layer of the capacitor 164. The same relation is applied toC1 and C2. Therefore, the material, the thickness, and the like of thegate insulating layer are desirably set as appropriate to satisfy theabove relation.

In the semiconductor device described in this embodiment, the node FGhas an effect similar to a floating gate of a floating-gate transistorin a flash memory or the like, but the node FG of this embodiment has afeature which is essentially different from that of the floating gate inthe flash memory or the like.

In a flash memory, since a potential supplied to a control gate is high,it is necessary to keep a proper distance between cells in order toprevent the potential from affecting a floating gate of the adjacentcell. This is one of inhibiting factors for high integration of thesemiconductor device. The factor is attributed to a basic principle of aflash memory, in which a tunneling current flows in applying a highelectrical field.

In contrast, the semiconductor device according to this embodiment isoperated by switching of a transistor including an oxide semiconductorand does not use the above-described principle of charge injection bytunneling current. In other words, a high electric field for chargeinjection is not necessary unlike a flash memory. Accordingly, it is notnecessary to consider an influence of a high electric field from acontrol gate on an adjacent cell, which facilitates high integration.

In addition, the semiconductor device according to the disclosedinvention is advantageous over a flash memory in that a high electricfield is not necessary and a large peripheral circuit (such as a boostercircuit) is not necessary. For example, the highest voltage applied tothe memory cell according to this embodiment (the difference between thehighest potential and the lowest potential supplied to terminals of thememory cell at the same time) can be 5 V or lower, preferably 3 V orlower in each memory cell in the case where two levels (one bit) of dataare written.

In the case where the dielectric constant ∈r1 of the insulating layerincluded in the capacitor 164 is different from the dielectric constant∈r2 of the insulating layer included in the transistor 160, C1 and C2can easily satisfy C1≧C2 while S1 which is the area of the insulatinglayer included in the capacitor 164 and S2 which is the area of aninsulating layer forming gate capacitance of the transistor 160 satisfy2×S2≧S1 (desirably S2≧S1). In other words, C1 can easily be made greaterthan or equal to C2 while the area of the insulating layer included inthe capacitor 164 is made small. Specifically, for example, a filmformed of a high-k material such as hafnium oxide or a stack of a filmformed of a high-k material such as hafnium oxide and a film formed ofan oxide semiconductor is used for the insulating layer including in thecapacitor 164 so that ∈r1 can be set to 10 or more, preferably 15 ormore, and silicon oxide is used for the insulating layer forming thegate capacitance of the transistor 160 so that ∈r2 can be set to 3 to 4.

Combination of such structures enables higher integration of thesemiconductor device according to the disclosed invention.

<Basic Circuit 2>

FIGS. 2A and 2B are circuit diagrams of memory cell arrays in each ofwhich the memory cells illustrated in FIG. 1A-1 are arranged in a matrixin two rows and two columns. The structures of memory cells 170 in FIGS.2A and 2B are similar to that in FIG. 1A-1. Note that the memory cellsin two columns share the source line SL in FIG. 2A, and the memory cellsin two rows share the source line SL in FIG. 2B.

When memory cells in two columns or two rows share the source line SL asin FIG. 2A or 2B, the number of signal lines connected to the memorycells 170 can be reduced to 3.5 (3+½) from 4 that is the number ofsignal lines in the case where the source line SL is not shared.

Note that without limitation to two columns (or two rows), pluralcolumns (or plural rows) such as three or more columns (or three or morerows) may share the source line SL. The number of columns (or rows)which share the source line SL may be determined as appropriate inconsideration of parasitic resistance and parasitic capacitancegenerated when the source line SL is shared. Further, the number ofcolumns (or rows) which share the source line SL is preferably large, inwhich case the number of signal lines connected to the memory cells 170can be reduced.

In FIGS. 2A and 2B, the source line SL is connected to a source lineswitching circuit 194. Here, the source line switching circuit 194 isconnected to a source line switching signal line SLC in addition to thesource line SL.

In semiconductor devices illustrated in FIGS. 2A and 2B, writing,holding, and reading of data are performed in a manner similar to thatin the case of FIGS. 1A-1, 1A-2, and 1B; therefore, the abovedescription can be referred to. Note that, a writing operation, forexample, in the case where either a power supply potential V_(DD) or aground potential GND is supplied to the node FG and in the case wheredata held at the time when a power supply potential V_(DD) is suppliedto the node FG is data “1” and data held at the time when a groundpotential GND is supplied to the node FG is data “0” will bespecifically described below. First, the potential of the write-readword line C connected to the memory cell 170 is set to GND and thepotential of the write word line OSG connected to the memory cell 170 isset to V_(DD), so that the memory cell 170 is selected. Accordingly, thepotential of the bit line BL is supplied to the node FG of the selectedmemory cell 170.

Here, in the case where a ground potential GND is supplied to the nodeFG (i.e., in the case where data “0” is held), a potential which allowsthe transistor 160 to be turned on is supplied to the gate electrode ofthe transistor 160. In that case, the potential of the source line SLneeds to be set to a ground potential GND in order to suppress anincrease in potential written to the node FG due to current flowing inthe bit line BL and the source line SL.

Thus, the signal path in the source line switching circuit 194 isswitched with a signal of the source line switching signal line SLC tosupply a ground potential GND to the source line SL.

The operation is characterized in that the potential of the source lineSL is set to a ground potential GND when data is written. This cansuppress generation of current flowing in the bit line BL and the sourceline SL even when a potential which allows the transistor 160 to beturned on is supplied to the node FG.

In the case where the memory cells 170 are arrayed as in FIGS. 2A and2B, it is necessary to read out data only from the intended memory cell170 when data is read. In order to read out data only from thepredetermined memory cell 170 and not to read out data from the othermemory cells 170 as described above, the memory cells 170 from whichdata is not read out need to be in a non-selected state.

For example, as described in <Basic Circuit 1>, in the case where eithera power supply potential V_(DD) or a ground potential GND is supplied tothe node FG and in the case where data held at the time when a powersupply potential V_(DD) is supplied to the node FG is data “1” and dataheld at the time when a ground potential GND is supplied to the node FGis data “0”, the source line SL is set to GND, the write-read word lineC is set to V_(DD), and the write word line OSG is set to GND, so thatthe memory cell 170 can be brought into a non-selected state.

When the write-read word line C is set to V_(DD), the potential of thenode FG is increased by V_(DD) due to capacitive coupling with thecapacitor 164. In the case where V_(DD) that is data “1” has beenwritten to the node FG, the potential of the node FG is increased byV_(DD) to be 2 V_(DD) (V_(DD)+V_(DD)=2 V_(DD)) and V_(gsp) is higherthan V_(th) _(_) _(p); accordingly, the p-channel transistor 160 isturned off. In contrast, in the case where GND that is data “0” has beenwritten to the node FG, the potential of the node FG is increased byV_(DD) to be V_(DD) (GND+V_(DD)=V_(DD)) and V_(gsp) is higher thanV_(th) _(_) _(p); accordingly, the p-channel transistor 160 is turnedoff In other words, by setting the write-read word line C to V_(DD), thetransistor 160 can be turned off, that is, the memory cell 170 can bebrought into a non-selected state regardless of the data held in thenode FG.

Note that if n-channel transistors are used as the reading transistors160, in the case where the potentials of gate electrodes of then-channel transistors are higher than the threshold voltage of thetransistors, not all memory cells can be turned off even by setting thewrite-read word line C to 0 V. Thus, a negative potential needs to besupplied to the write-read word line C in a non-selected row in order tobring the memory cells into a non-selected state. In contrast, in thesemiconductor device described in this embodiment, p-channel transistorsare used as the reading transistors. Thus, memory cells in anon-selected row can be turned off by setting the write-read word line Cin a non-selected row to a high potential. Accordingly, a power supplygenerating a negative potential does not need to be provided for thememory cell. As a result, power consumption can be reduced and thesemiconductor device can be downsized.

As described above, in the semiconductor devices having the circuitconfigurations illustrated in FIGS. 2A and 2B, the area of the memorycell array can be reduced by sharing the source line SL by pluralcolumns (or plural rows). Accordingly, the die size can be reduced.Moreover, the reduction in the die size allows cost reduction inmanufacturing the semiconductor device or improvement in yield.

Application Example 1

Next, a more specific circuit configuration to which the circuitillustrated in FIGS. 1A-1, 1A-2 and 1B is applied and its operationthereof will be described with reference to FIG. 3 and FIG. 4. Note thatthe case where an n-channel transistor is used as a writing transistor(the transistor 162) and a p-channel transistor is used as a readingtransistor (the transistor 160) will be described below as an example.Note that in the circuit diagram illustrated in FIG. 3, wirings withslashes are bus signal lines.

FIG. 3 is an example of a circuit diagram of a semiconductor deviceincluding (m×n) memory cells 170. The configuration of the memory cells170 in FIG. 3 is similar to that in FIG. 1A-1.

The semiconductor device illustrated in FIG. 3 includes m (m is aninteger greater than or equal to 2) write word lines OSG, m write-readword lines C, n (n is an integer greater than or equal to 2) bit linesBL, a source line SL, a memory cell array having the memory cells 170arranged in a matrix of m (rows)×n (columns), a step-up circuit 180, afirst driver circuit 182 including an address decoder, a second drivercircuit 192 including a row driver, a third driver circuit 190 includinga page buffer, a fourth driver circuit 184 including a controller, afifth driver circuit 186 including an input-output control circuit, andthe source line switching circuit 194. The number of driver circuits isnot limited to the number in FIG. 3. Driver circuits having variousfunctions may be combined. Alternatively, functions of each drivercircuit may be separated for other driver circuits.

In the semiconductor device illustrated in FIG. 3, the first drivercircuit 182 includes an address decoder. The address decoder is acircuit which decodes an address selection signal line A and outputs thedecoded address selection signal to a row selection signal line RADR anda page buffer address selection signal line PBADR. The address selectionsignal line A corresponds to a terminal to which a row address selectionsignal of the memory cells 170 and a page buffer address selectionsignal are input. One or more address selection signal lines A areprovided depending on the numbers of columns and rows of the memorycells 170 or the structure of the page buffer. The row selection signalline RADR is a signal line which specifies the row address of memorycells. The page buffer address selection signal line PBADR is a signalline which specifies the page buffer address.

The second driver circuit 192 includes a row driver. The row driveroutputs a row selection signal of the memory cells 170, a signal to thewrite word line OSG, and a signal to the write-read word line C on thebasis of a signal output to the row selection signal line RADR from theaddress decoder included in the first driver circuit 182.

The step-up circuit 180 is connected to the second driver circuit 192through a wiring VH-L and is configured to step up a constant potential(e.g., a power supply potential V_(DD)) which is input from the step-upcircuit 180 and to output a potential (VH) higher than the constantpotential to the second driver circuit 192. In order to prevent apotential written to the node FG of the memory cell 170 from beingdecreased by the threshold voltage (V_(th) _(_) _(OS)) of the transistor162 that is a writing transistor, the potential of the write word lineOSG needs to be set higher than the sum of the potential of the bit lineBL and V_(th) _(_) _(OS). Thus, for example, when the power supplypotential V_(DD) is written to the node FG, VH is set higher than orequal to (V_(DD)+V_(th) _(_) _(OS)). Note that if a decrease in thepotential written to the node FG by V_(th) _(_) _(OS) does not cause anyproblem, the step-up circuit 180 is not necessarily provided.

The third driver circuit 190 includes a page buffer. The page bufferfunctions as both a data latch and a sense amplifier. The page bufferfunctions as a data latch as follows: the page buffer temporarily holdsdata output from an internal data input-output signal line INTDIO or thebit line BL and outputs the held data to the internal data input-outputsignal line INTDIO or the bit line BL. The page buffer functions as asense amplifier as follows: the page buffer senses the bit line BL towhich data is output from the memory cell when data is read.

The fourth driver circuit 184 is a circuit which includes a controllerand generates signals for controlling the first driver circuit 182, thesecond driver circuit 192, the third driver circuit 190, the fifthdriver circuit 186, the source line switching circuit 194, and thestep-up circuit 180, from a signal from a chip-enable bar signal lineCEB, a write-enable bar signal line WEB, or a read-enable bar signalline REB.

The chip-enable bar signal line CEB is a signal line for outputting aselection signal for the entire circuit, and accepts an input signal andoutputs an output signal only when it is active. The write-enable barsignal line WEB is a signal line for outputting a signal which allowslatch data of the page buffer in the third driver circuit 190 to bewritten to the memory cell array. The read-enable bar signal line REB isa signal line for outputting a signal which allows data of the memorycell array to be read out. The fourth driver circuit 184 is connected tothe step-up circuit 180 through a step-up circuit control signal lineBCC. The step-up circuit control signal line BCC is a wiring fortransmitting a control signal of the step-up circuit which is outputfrom the controller in the fourth driver circuit 184. No step-up circuitcontrol signal line BCC or plural step-up circuit control signal linesBCC are provided depending on the circuit configuration. In addition,the fourth driver circuit 184 is connected to the third driver circuit190 through a page buffer control signal line PBC. The page buffercontrol signal line PBC is a wiring for transmitting a control signal ofthe page buffer which is output from the controller in the fourth drivercircuit 184. No page buffer control signal line PBC or plural pagebuffer control signal lines PBC are provided depending on the circuitconfiguration. In addition, the fourth driver circuit 184 is connectedto the second driver circuit 192 through a row driver control signalline RDRVC. In addition, the fourth driver circuit 184 is connected tothe source line switching circuit 194 through the source line switchingsignal line SLC.

The source line switching circuit 194 is a circuit which switches thepotential of the source line SL on the basis of a source line switchingsignal from the controller in the fourth driver circuit 184. The sourceline switching circuit 194 may have a function of switching thepotential of the source line SL, and a multiplexer, an inverter, or thelike may be used. The source line switching signal line SLC is a wiringfor transmitting a signal which is for switching the potential of thesource line SL and is output from the controller in the fourth drivercircuit 184. One or more signal lines are provided depending on thecircuit configuration.

The fifth driver circuit 186 includes an input-output control circuit.The input-output control circuit is a circuit for outputting an inputsignal from a data input-output signal line DIO to the internal datainput-output signal line INTDIO or outputting an input signal from theinternal data input-output signal line INTDIO to the data input-outputsignal line DIO. A terminal of the data input-output signal line DIO isa terminal to which external data is input or from which memory data isoutput to the outside. One or more signal lines are provided dependingon the circuit configuration. The internal data input-output signal lineINTDIO is a signal line for inputting an output signal from theinput-output control circuit to the page buffer or inputting an outputsignal from the page buffer to the input-output control circuit. One ormore signal lines are provided depending on the circuit configuration.Further, the data input-output signal line DIO may be divided into adate-input signal line and a data output signal line.

In the semiconductor device illustrated in FIG. 3, writing, holding, andreading of data are basically similar to those in the case of FIGS.1A-1, 1A-2, and 1B and FIGS. 2A and 2B. FIG. 4 is an example of a timingchart for the writing and reading operations of the semiconductor devicein FIG. 3. Specifically, an example of an operation of writing latchdata of the page buffer to the memory cell array and an example of anoperation of reading out data written to the memory cell array andlatching the data in the page buffer will be described. In the timingchart, CEB, WEB, and the like denote the wirings to which the potentialsin the timing chart are applied. Wirings having a similar function aredistinguished by “1”, “m”, “n”, and the like added to the end of theirnames. Note that the disclosed invention is not limited to thearrangement described below. Although CEB, WEB, and REB are active whena potential of “Low” is input in the circuit configuration described inthis embodiment, a circuit may be employed in which CEB, WEB, and REBare active when a potential of “High” is input.

The timing chart shows the relationship among the potentials of thewirings in the case where data “1” is written to the memory cell in thefirst row and the first column, data “0” is written to the memory cellin the first row and the n-th column, data “0” is written to the memorycell in the m-th row and the first column, and data “1” is written tothe memory cell in the m-th row and the n-th column in the memory cellsin m rows and n columns, and then the written data in all the abovememory cells is read out.

In a writing period, first, the chip-enable bar signal line CEB is setto a low potential and an address of the memory cell 170 to which datais written is specified by a signal from the address selection signalline A. After that, the write-enable bar signal line WEB is set to a lowpotential. Thus, writing is performed. The page buffer outputs latchdata which is writing data to the bit line BL. The row driver outputs ahigh potential to the write word line OSG in a selected row and thewrite-read word line C in a non-selected row and outputs a low potentialto the write word line in a non-selected row and the write-read wordline C in a selected row.

In the writing period, writing data is output to the bit line BL fromthe page buffer in accordance with the timing of row selection. The bitline BL in the case of writing of data “1” has a high potential, whereasthe bit line BL in the case of writing of data “0” has a low potential.Note that the signal input period of the bit line BL is set so as to belonger than that of the write word line OSG in a selected row and thewrite-read word line C in a selected row for the following reason: ifthe signal input period of the bit line BL is short, incorrect writingof data to the memory cell might be caused.

Note that in the case where a ground potential GND is supplied to thenode FG in the writing period, the potential of the source line SL isset to a ground potential GND in order to prevent current flowing in thebit line BL and the source line SL. This driving is performed byswitching the signal path in the source line switching circuit 194 witha signal from the source line switching signal line SLC.

In a reading period, first, the chip-enable bar signal line CEB is setto a low potential and an address of the memory cell 170 from which datais read is specified by a signal from the address selection signal lineA. After that, the read-enable bar signal line REB is set to a lowpotential. Thus, reading is performed. The page buffer latches datawhich is read out to the bit line BL from the memory cell. The rowdriver outputs a low potential to the write-read word line C in aselected row and outputs a high potential to the write-read word line Cin a non-selected row. The write word line OSG has a low potentialregardless of a selected state or a non-selected state. The source lineswitching circuit 194 outputs a high potential to the source line SL.

In the reading period, a potential corresponding to the data which hasbeen written to the memory cell 170 is output to the bit line BL inaccordance with the timing of row selection. The bit line BL has a lowpotential when data “1” has been written to the memory cell, whereas thebit line BL has a high potential when data “0” has been written to thememory cell.

In a standby and data holding period, the chip-enable bar signal lineCEB is set to a high potential to make the entire circuit illustrated inFIG. 3 non-active. In this case, neither writing nor reading isperformed; thus, control signals for WEB, REB, and the like may haveeither a high potential or a low potential.

Note that the slashes in the timing chart of FIG. 4 means that thecorresponding line may have either a high potential or a low potential.

As described above, in the semiconductor device having the circuitconfiguration illustrated in FIG. 3, the area of the memory cell arraycan be reduced by sharing the source line SL with plural columns.Accordingly, the die size can be reduced. Moreover, the reduction in thedie size allows cost reduction in manufacturing the semiconductor deviceor improvement in yield.

Further, in the semiconductor device illustrated in FIG. 3, memory cellsin a non-selected row need to be turned off when data is read. In thesemiconductor device described in this embodiment, p-channel transistorsare used as the reading transistors. Thus, memory cells in anon-selected row can be turned off by setting the write-read word line Cof the non-selected row to a high potential (e.g., a power supplypotential). Accordingly, a power supply generating a negative potentialdoes not need to be provided for the memory cell. As a result, powerconsumption can be reduced and the semiconductor device can bedownsized.

Note that the operation method, the operation voltage, and the likerelating to the semiconductor device of an embodiment of the disclosedinvention are not limited to the above description and can be changedappropriately in accordance with an embodiment in which the operation ofthe semiconductor device is implemented.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

Embodiment 2

In this embodiment, a structure and a manufacturing method of asemiconductor device according to one embodiment of the disclosedinvention will be described with reference to FIGS. 5A and 5B, FIGS. 6Ato 6G, FIGS. 7A to 7E, FIGS. 8A to 8D, FIGS. 9A to 9D, and FIGS. 10A to10C.

<Cross-Sectional Structure and Planar Structure of Semiconductor Device>

FIGS. 5A and 5B illustrate an example of a structure of a semiconductordevice. FIG. 5A illustrates a cross section of the semiconductor device,and FIG. 5B illustrates a plan view of the semiconductor device. FIG. 5Acorresponds to a cross section along line A1-A2 and line B1-B2 in FIG.5B. The semiconductor device illustrated in FIGS. 5A and 5B includes atransistor 160 including a first semiconductor material in a lowerportion, and a transistor 162 including a second semiconductor materialin an upper portion. It is preferable that the first semiconductormaterial and the second semiconductor material be different from eachother. For example, a semiconductor material other than an oxidesemiconductor can be used as the first semiconductor material, and anoxide semiconductor can be used as the second semiconductor material.The semiconductor material other than an oxide semiconductor can be, forexample, silicon, germanium, silicon germanium, silicon carbide, indiumphosphide, gallium arsenide, or the like and is preferably singlecrystalline. An organic semiconductor material or the like may be used.A transistor including such a semiconductor material other than an oxidesemiconductor can operate at high speed easily. On the other hand, atransistor including an oxide semiconductor can hold charge for a longtime owing to its characteristics. The semiconductor device illustratedin FIGS. 5A and 5B can be used as a memory cell.

The technical feature of the invention disclosed herein lies in the useof a semiconductor material with which off-state current can besufficiently reduced, such as an oxide semiconductor, in the transistor162 in order to hold data. Therefore, it is not necessary to limitspecific conditions, such as a material, a structure, or the like of thesemiconductor device, to those given here.

The transistor 160 in FIGS. 5A and 5B includes a channel formationregion 134 provided in a semiconductor layer over a semiconductorsubstrate 500, impurity regions 132 (also referred to as a source regionand a drain region) with the channel formation region 134 providedtherebetween, a gate insulating layer 122 a provided over the channelformation region 134, and a gate electrode 128 a provided over the gateinsulating layer 122 a so as to overlap with the channel formationregion 134. Note that a transistor whose source electrode and drainelectrode are not illustrated in a drawing may be referred to as atransistor for the sake of convenience. Further, in such a case, indescription of a connection of a transistor, a source region and asource electrode are collectively referred to as a “source electrode”,and a drain region and a drain electrode are collectively referred to asa “drain electrode”. In other words, in this specification, the term“source electrode” may include a source region.

Further, a conductive layer 128 b is connected to an impurity region 126provided in the semiconductor layer over the semiconductor substrate500. Here, the conductive layer 128 b functions as a source electrode ora drain electrode of the transistor 160. In addition, an impurity region130 is provided between the impurity region 132 and the impurity region126. Further, insulating layers 136, 138, and 140 are provided so as tocover the transistor 160. Note that for high integration, it ispreferable that, as in FIGS. 5A and 5B, the transistor 160 does not havea sidewall insulating layer. On the other hand, when importance is puton the characteristics of the transistor 160, sidewall insulating layersmay be provided on side surfaces of the gate electrode 128 a, and theimpurity region 132 may include regions with different impurityconcentrations.

The transistor 162 in FIGS. 5A and 5B includes an oxide semiconductorlayer 144 which is provided over the insulating layer 140 and the like;a source electrode (or a drain electrode) 142 a and a drain electrode(or a source electrode) 142 b which are electrically connected to theoxide semiconductor layer 144; a gate insulating layer 146 which coversthe oxide semiconductor layer 144, the source electrode 142 a, and thedrain electrode 142 b; and a gate electrode 148 a which is provided overthe gate insulating layer 146 so as to overlap with the oxidesemiconductor layer 144.

Here, the oxide semiconductor layer 144 is preferably a purified oxidesemiconductor layer by sufficiently removing impurities such as hydrogenor sufficiently supplying oxygen. Specifically, the hydrogenconcentration of the oxide semiconductor layer 144 is 5×10¹⁹ atoms/cm³or lower, preferably 5×10¹⁸ atoms/cm³ or lower, more preferably 5×10¹⁷atoms/cm³ or lower. Note that the hydrogen concentration of the oxidesemiconductor layer 144 is measured by secondary ion mass spectrometry(SIMS). In the oxide semiconductor layer 144 which is purified bysufficiently reducing the concentration of hydrogen therein and in whichdefect levels in an energy gap due to oxygen deficiency are reduced bysupplying a sufficient amount of oxygen, the carrier concentration islower than 1×10¹²/cm³, preferably lower than 1×10¹¹/cm³, more preferablylower than 1.45×10¹⁰/cm³. For example, the off-state current (here,current per micrometer (μm) of channel width) at room temperature (25°C.) is lower than or equal to 100 zA (1 zA (zeptoampere) is 1×10⁻²¹ A),preferably lower than or equal to 10 zA. In this manner, by using anoxide semiconductor which is made to be an i-type (intrinsic) oxidesemiconductor or a substantially i-type oxide semiconductor, thetransistor 162 which has extremely favorable off-state currentcharacteristics can be obtained.

Note that although the transistor 162 in FIGS. 5A and 5B includes theoxide semiconductor layer 144 which is processed into an island shape inorder to suppress leakage current between elements which is caused dueto miniaturization, the oxide semiconductor layer 144 which is notprocessed into an island shape may be employed. In the case where theoxide semiconductor layer is not processed into an island shape,contamination of the oxide semiconductor layer 144 due to etching in theprocessing can be prevented.

A capacitor 164 in FIGS. 5A and 5B includes the drain electrode 142 b,the gate insulating layer 146, and a conductive layer 148 b. In otherwords, the drain electrode 142 b functions as one electrode of thecapacitor 164, and the conductive layer 148 b functions as the otherelectrode of the capacitor 164. Such a structure allows sufficientcapacitance to be secured. Further, insulation between the drainelectrode 142 b and the conductive layer 148 b can be sufficientlysecured by stacking the oxide semiconductor layer 144 and the gateinsulating layer 146. The capacitor 164 may be omitted in the case wherea capacitor is not needed.

In this embodiment, the transistor 162 and the capacitor 164 areprovided so that at least parts of the transistor 162 and the capacitor164 overlap with the transistor 160. Such a planar layout allows thedegree of integration to be increased. For example, when F is used toexpress the minimum feature size, the area of a memory cell can beexpressed as 15F² to 25F².

An insulating layer 150 is provided over the transistor 162 and thecapacitor 164. A wiring 154 is provided in an opening formed in the gateinsulating layer 146 and the insulating layer 150. The wiring 154 is awiring which connects one memory cell to another memory cell andcorresponds to the bit line BL in the circuit diagrams in FIGS. 2A and2B. The wiring 154 is connected to the impurity region 126 through thesource electrode 142 a and the conductive layer 128 b. The abovestructure allows a reduction in the number of wirings as compared to astructure in which the source region or the drain region in thetransistor 160 and the source electrode 142 a in the transistor 162 areconnected to different wirings. Thus, the degree of integration of asemiconductor device can be increased.

Since the conductive layer 128 b is provided, the position where theimpurity region 126 and the source electrode 142 a are connected to eachother and the position where the source electrode 142 a and the wiring154 are connected to each other can overlap with each other. Such aplanar layout makes it possible to suppress an increase in the elementarea due to contact regions. In other words, the degree of integrationof the semiconductor device can be increased.

<Manufacturing Method of SOI Substrate>

Next, an example of a manufacturing method of an SOI substrate used formanufacturing the above semiconductor device will be described withreference to FIGS. 6A to 6G.

First, a semiconductor substrate 500 is prepared as a base substrate(see FIG. 6A). As the semiconductor substrate 500, a semiconductorsubstrate such as a single crystal silicon substrate or a single crystalgermanium substrate can be used. As the semiconductor substrate, a solargrade silicon (SOG-Si) substrate or the like may be used. Apolycrystalline semiconductor substrate may be used. Manufacturing costin the case of using a SOG-Si substrate, a polycrystalline semiconductorsubstrate, or the like can be lower than that in the case of using asingle crystal silicon substrate or the like.

Note that instead of the semiconductor substrate 500, a variety of glasssubstrates used in the electronics industry, such as substrates made ofaluminosilicate glass, aluminoborosilicate glass, and bariumborosilicate glass; a quartz substrate; a ceramic substrate; or asapphire substrate can be used. Further, a ceramic substrate whichcontains silicon nitride and aluminum nitride as its main components andwhose coefficient of thermal expansion is close to that of silicon maybe used.

A surface of the semiconductor substrate 500 is preferably cleaned inadvance. Specifically, the semiconductor substrate 500 is preferablysubjected to ultrasonic cleaning with a hydrochloric acid/hydrogenperoxide mixture (HPM), a sulfuric acid/hydrogen peroxide mixture (SPM),an ammonium hydrogen peroxide mixture (APM), diluted hydrofluoric acid(DHF), a mixed solution of hydrofluoric acid, hydrogen peroxide water,and pure water (FPM), or the like.

Next, a bond substrate is prepared. Here, a single crystal semiconductorsubstrate 510 is used as the bond substrate (see FIG. 6B). Note thatalthough a substrate whose crystallinity is single crystal is used asthe bond substrate here, the crystallinity of the bond substrate is notnecessarily limited to single crystal.

For example, as the single crystal semiconductor substrate 510, a singlecrystal semiconductor substrate formed using an element of Group 14,such as a single crystal silicon substrate, a single crystal germaniumsubstrate, or a single crystal silicon germanium substrate, can be used.Further, a compound semiconductor substrate using gallium arsenide,indium phosphide, or the like can be used. Typical examples ofcommercially available silicon substrates are circular siliconsubstrates which are 5 inches (125 mm) in diameter, 6 inches (150 mm) indiameter, 8 inches (200 mm) in diameter, 12 inches (300 mm) in diameter,and 16 inches (400 mm) in diameter. Note that the shape of the singlecrystal semiconductor substrate 510 is not limited to circular, and thesingle crystal semiconductor substrate 510 may be a substrate which hasbeen processed into, for example, a rectangular shape or the like.Further, the single crystal semiconductor substrate 510 can be formed bya Czochralski (CZ) method or a Floating Zone (FZ) method.

An oxide film 512 is formed on a surface of the single crystalsemiconductor substrate 510 (see FIG. 6C). In view of removal ofcontamination, it is preferable that the surface of the single crystalsemiconductor substrate 510 be cleaned with a hydrochloric acid/hydrogenperoxide mixture (HPM), a sulfuric acid/hydrogen peroxide mixture (SPM),an ammonium hydrogen peroxide mixture (APM), diluted hydrofluoric acid(DHF), FPM (a mixed solution of hydrofluoric acid, hydrogen peroxidewater, and pure water), or the like before the formation of the oxidefilm 512. Dilute hydrofluoric acid and ozone water may be dischargedalternately for cleaning.

The oxide film 512 can be formed with, for example, a single layer or astacked layer of a silicon oxide film, a silicon oxynitride film, andthe like. As a manufacturing method of the oxide film 512, a thermaloxidation method, a CVD method, a sputtering method, or the like can beused. In the case where the oxide film 512 is formed by a CVD method, asilicon oxide film is preferably formed using organosilane such astetraethoxysilane (abbreviation: TEOS) (chemical formula: Si(OC₂H₅)₄),so that favorable bonding can be achieved.

In this embodiment, the oxide film 512 (here, a SiO_(x) film) is formedby performing thermal oxidation treatment on the single crystalsemiconductor substrate 510. The thermal oxidation treatment ispreferably performed in an oxidizing atmosphere to which a halogen isadded.

For example, thermal oxidation treatment of the single crystalsemiconductor substrate 510 is performed in an oxidizing atmosphere towhich chlorine (Cl) is added, whereby the oxide film 512 can be formedthrough chlorine oxidation. In this case, the oxide film 512 is a filmcontaining chlorine atoms. By such chlorine oxidation, heavy metal(e.g., Fe, Cr, Ni, or Mo) that is an extrinsic impurity is trapped andchloride of the metal is formed and then removed to the outside; thus,contamination of the single crystal semiconductor substrate 510 can bereduced.

Note that the halogen atoms contained in the oxide film 512 are notlimited to chlorine atoms. A fluorine atom may be contained in the oxidefilm 512. As a method for fluorine oxidation of the surface of thesingle crystal semiconductor substrate 510, a method in which the singlecrystal semiconductor substrate 510 is soaked in an HF solution and thensubjected to thermal oxidation treatment in an oxidizing atmosphere, amethod in which thermal oxidation treatment is performed in an oxidizingatmosphere to which NF₃ is added, or the like can be used.

Next, ions are accelerated by an electric field and the single crystalsemiconductor substrate 510 is irradiated with the ions, and the ionsare added to the single crystal semiconductor substrate 510, whereby anembrittled region 514 where the crystal structure is damaged is formedin the single crystal semiconductor substrate 510 at a predetermineddepth (see FIG. 6D).

The depth at which the embrittled region 514 is formed can be adjustedby the kinetic energy, mass, charge, or incidence angle of the ions, orthe like. The embrittled region 514 is formed at approximately the samedepth as the average penetration depth of the ions. Therefore, thethickness of the single crystal semiconductor layer to be separated fromthe single crystal semiconductor substrate 510 can be adjusted with thedepth at which the ions are added. For example, the average penetrationdepth may be controlled such that the thickness of a single crystalsemiconductor layer is approximately 10 nm to 500 nm, preferably, 50 nmto 200 nm.

The above ion irradiation treatment can be performed with an ion-dopingapparatus or an ion-implantation apparatus. As a typical example of theion-doping apparatus, there is a non-mass-separation type apparatus inwhich plasma excitation of a process gas is performed and an object tobe processed is irradiated with all kinds of ion species generated. Inthis apparatus, the object to be processed is irradiated with ionspecies of plasma without mass separation. In contrast, an ionimplantation apparatus is a mass-separation apparatus. In theion-implantation apparatus, mass separation of ion species of plasma isperformed and the object to be processed is irradiated with ion specieshaving predetermined masses.

In this embodiment, an example is described in which an ion dopingapparatus is used to add hydrogen to the single crystal semiconductorsubstrate 510. A gas containing hydrogen is used as a source gas. As forions used for the irradiation, the proportion of H₃ ⁺ is preferably sethigh. Specifically, it is preferable that the proportion of H₃ ⁺ be set50% or higher (more preferably, 80% or higher) with respect to the totalamount of H⁺, H₂ ⁺, and H₃ ⁺. With a high proportion of H₃ ⁺, theefficiency of ion irradiation can be improved.

Note that ions to be added are not limited to ions of hydrogen. Ions ofhelium or the like may be added. Further, ions to be added are notlimited to one kind of ions, and plural kinds of ions may be added. Forexample, in the case of performing irradiation with hydrogen and heliumconcurrently using an ion-doping apparatus, the number of steps can bereduced as compared to the case of performing irradiation with hydrogenand helium in different steps, and surface roughness of a single crystalsemiconductor layer to be formed later can be suppressed.

Note that heavy metal may also be added when the embrittled region 514is formed with the ion doping apparatus; however, the ion irradiation isperformed through the oxide film 512 containing halogen atoms, wherebycontamination of the single crystal semiconductor substrate 510 due tothe heavy metal can be prevented.

Then, the semiconductor substrate 500 and the single crystalsemiconductor substrate 510 are disposed to face each other and are madeto be closely attached to each other with the oxide film 512therebetween. As a result, the semiconductor substrate 500 and thesingle crystal semiconductor substrate 510 are bonded to each other (seeFIG. 6E). Note that an oxide film or a nitride film may be formed on thesurface of the semiconductor substrate 500 to which the single crystalsemiconductor substrate 510 is attached.

When bonding is performed, it is preferable that pressure greater thanor equal to 0.001 N/cm² and less than or equal to 100 N/cm², e.g., apressure greater than or equal to 1 N/cm² and less than or equal to 20N/cm², be applied to one part of the semiconductor substrate 500 or onepart of the single crystal semiconductor substrate 510. When the bondingsurfaces are made close to each other and disposed in close contact witheach other by applying a pressure, a bonding between the semiconductorsubstrate 500 and the oxide film 512 is generated at the part where theclose contact is made, and from that part, the bonding spontaneouslyspreads to almost the entire area. This bonding is performed under theaction of the Van der Waals force or hydrogen bonding and can beperformed at room temperature.

Note that before the single crystal semiconductor substrate 510 and thesemiconductor substrate 500 are bonded to each other, surfaces to bebonded to each other are preferably subjected to surface treatment.Surface treatment can improve the bonding strength at the interfacebetween the single crystal semiconductor substrate 510 and thesemiconductor substrate 500.

As the surface treatment, wet treatment, dry treatment, or a combinationof wet treatment and dry treatment can be used. Alternatively, wettreatment may be used in combination with different wet treatment or drytreatment may be used in combination with different dry treatment.

Note that heat treatment for increasing the bonding strength may beperformed after bonding. This heat treatment is performed at atemperature at which separation at the embrittled region 514 does notoccur (for example, a temperature higher than or equal to roomtemperature and lower than 400° C.). Bonding of the semiconductorsubstrate 500 and the oxide film 512 may be performed while heating themat a temperature in this range. The heat treatment can be performedusing a diffusion furnace, a heating furnace such as a resistanceheating furnace, a rapid thermal annealing (RTA) apparatus, a microwaveheating apparatus, or the like. The above temperature condition ismerely an example, and an embodiment of the disclosed invention shouldnot be construed as being limited to this example.

Next, heat treatment is performed for separation of the single crystalsemiconductor substrate 510 at the embrittled region, whereby a singlecrystal semiconductor layer 516 is formed over the semiconductorsubstrate 500 with the oxide film 512 interposed therebetween (FIG. 6F).

Note that it is desirable that the temperature for heat treatment in theseparation be as low as possible. This is because as the temperature inthe separation is low, generation of roughness on the surface of thesingle crystal semiconductor layer 516 can be suppressed. Specifically,the temperature for the heat treatment in the separation may be higherthan or equal to 300° C. and lower than or equal to 600° C. and the heattreatment is more effective when the temperature is higher than or equalto 400° C. and lower than or equal to 500° C.

Note that after the single crystal semiconductor substrate 510 isseparated, the single crystal semiconductor layer 516 may be subjectedto heat treatment at 500° C. or higher so that concentration of hydrogenremaining in the single crystal semiconductor layer 516 is reduced.

Next, the surface of the single crystal semiconductor layer 516 isirradiated with laser light, whereby a single crystal semiconductorlayer 518 whose surface planarity is improved and in which defects isreduced are formed (see FIG. 6G). Note that instead of the laser lightirradiation treatment, heat treatment may be performed.

Although the irradiation treatment with the laser light is performedimmediately after the heat treatment for separation of the singlecrystal semiconductor layer 516 in this embodiment, one embodiment ofthe present invention is not construed as being limited to this. Thelaser light irradiation treatment may be performed after the heattreatment for splitting the single crystal semiconductor layer 516 andetching treatment for removing a region including many defects at thesurface of the single crystal semiconductor layer 516 are performed inthis order. Alternatively, the laser light irradiation treatment may beperformed after the surface planarity of the single crystalsemiconductor layer 516 is improved. Note that the etching treatment maybe either wet etching or dry etching. Further, after the irradiationwith laser light is performed as described above, a step of reducing thethickness of the single crystal semiconductor layer 516 may beperformed. In order to reduce the thickness of the single crystalsemiconductor layer 516, either or both of dry etching and wet etchingmay be employed.

Through the above steps, an SOI substrate including the single crystalsemiconductor layer 518 with favorable characteristics can be obtained(see FIG. 6G).

<Manufacturing Method of Semiconductor Device>

Next, a manufacturing method of a semiconductor device including theabove SOI substrate will be described with reference to FIGS. 7A to 7E,FIGS. 8A to 8D, FIGS. 9A to 9D, and FIGS. 10A to 10C.

<Manufacturing Method of Transistor in Lower Portion>

First, a manufacturing method of the transistor 160 in the lower portionwill be described with reference to FIGS. 7A to 7E and FIGS. 8A to 8D.Note that FIGS. 7A to 7E and FIGS. 8A to 8D illustrate part of the SOIsubstrate formed by the method illustrated in FIGS. 6A to 6G, and arecross-sectional views illustrating the transistor in the lower portionillustrated in FIG. 5A.

First, the single crystal semiconductor layer 518 is patterned to havean island shape, so that a semiconductor layer 120 is formed (see FIG.7A). Note that before or after this step, an impurity element impartingn-type conductivity or an impurity element imparting p-type conductivitymay be added to the semiconductor layer in order to control thethreshold voltage of the transistor. In the case where silicon is usedas the semiconductor, phosphorus, arsenic, or the like can be used as animpurity element imparting n-type conductivity. On the other hand,boron, aluminum, gallium, or the like can be used as an impurity elementimparting p-type conductivity.

Next, an insulating layer 122 is formed so as to cover the semiconductorlayer 120 (see FIG. 7B). The insulating layer 122 is to be a gateinsulating layer later. For example, the insulating layer 122 can beformed by performing heat treatment (e.g., thermal oxidation treatment,thermal nitridation treatment, or the like) on a surface of thesemiconductor layer 120. High-density plasma treatment may be employedinstead of heat treatment. The high-density plasma treatment can beperformed using, for example, a mixed gas of a rare gas such as He, Ar,Kr, or Xe and any of oxygen, nitrogen oxide, ammonia, nitrogen, orhydrogen. Needless to say, the insulating layer may be formed by a CVDmethod, a sputtering method, or the like. The insulating layer 122preferably has a single-layer structure or a stacked structure includingany of silicon oxide, silicon oxynitride, silicon nitride, hafniumoxide, aluminum oxide, tantalum oxide, yttrium oxide, hafnium silicate(HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate to which nitrogen is added(HfSi_(x)O_(y)N_(z) (x>0, y>0, z>0)), hafnium aluminate to whichnitrogen is added (HfAl_(x)O_(y)N_(z) (x>0, y>0, z>0)), and the like.The thickness of the insulating layer 122 can be, for example, greaterthan or equal to 1 nm and less than or equal to 100 nm, preferablygreater than or equal to 10 nm and less than or equal to 50 nm. In thisembodiment, a single layer of an insulating layer containing siliconoxide is formed by a plasma CVD method.

Next, a mask 124 is formed over the insulating layer 122 and an impurityelement imparting one conductivity type is added to the semiconductorlayer 120, so that the impurity region 126 is formed (see FIG. 7C). Notethat here, the mask 124 is removed after the impurity element is added.

Next, a mask is formed over the insulating layer 122 and a region of theinsulating layer 122 that overlaps with the impurity region 126 ispartly removed, so that the gate insulating layer 122 a is formed (seeFIG. 7D). Part of the insulating layer 122 can be removed by etchingsuch as wet etching or dry etching.

Next, a conductive layer for forming a gate electrode (including awiring formed using the same layer as the gate electrode) is formed overthe gate insulating layer 122 a and is processed, so that the gateelectrode 128 a and the conductive layer 128 b are formed (see FIG. 7E).

The conductive layer used for the gate electrode 128 a and theconductive layer 128 b can be formed using a metal material such asaluminum, copper, titanium, tantalum, or tungsten. The conductive layermay be formed using a semiconductor material such as polycrystallinesilicon. There is no particular limitation on the method for forming thelayer containing a conductive material, and a variety of film formationmethods such as an evaporation method, a CVD method, a sputteringmethod, or a spin coating method can be employed. The conductive layermay be processed by etching using a resist mask.

Next, an impurity element imparting one conductivity type is added tothe semiconductor layer with the use of the gate electrode 128 a and theconductive layer 128 b as masks, so that the channel formation region134, the impurity region 132, and the impurity region 130 are formed(see FIG. 8A). Here, an impurity element such as boron (B) or aluminum(A) is added in order to form a p-channel transistor. Here, theconcentration of the impurity element to be added can be set asappropriate. In addition, after the impurity element is added, heattreatment for activation is performed. Here, the concentration of theimpurity element in the impurity region is increased in the followingorder: the impurity region 126, the impurity region 132, and theimpurity region 130.

Next, the insulating layer 136, the insulating layer 138, and theinsulating layer 140 are formed so as to cover the gate insulating layer122 a, the gate electrode 128 a, and the conductive layer 128 b (seeFIG. 8B).

The insulating layer 136, the insulating layer 138, and the insulatinglayer 140 can be formed using a material including an inorganicinsulating material such as silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, or aluminum oxide. The insulating layer136, the insulating layer 138, and the insulating layer 140 areparticularly preferably formed using a low dielectric constant (low-k)material, because capacitance due to overlapping electrodes or wiringscan be sufficiently reduced. Note that the insulating layer 136, theinsulating layer 138, and the insulating layer 140 may be porousinsulating layers formed using any of these materials. Since the porousinsulating layer has low dielectric constant as compared to a denseinsulating layer, capacitance due to electrodes or wirings can befurther reduced. Alternatively, the insulating layer 136, the insulatinglayer 138, and the insulating layer 140 can be formed using an organicinsulating material such as polyimide or acrylic. In this embodiment,the case of using silicon oxynitride for the insulating layer 136,silicon nitride oxide for the insulating layer 138, and silicon oxidefor the insulating layer 140 will be described. A stacked structure ofthe insulating layer 136, the insulating layer 138, and the insulatinglayer 140 is employed here; however, one embodiment of the disclosedinvention is not limited to this. A single-layer structure, a stackedstructure of two layers, or a stacked structure of four or more layersmay also be used.

Next, the insulating layer 138 and the insulating layer 140 aresubjected to chemical mechanical polishing (CMP) treatment, or etchingtreatment, so that the insulating layer 138 and the insulating layer 140are flattened (see FIG. 8C). Here, CMP treatment is performed until theinsulating layer 138 is partly exposed. In the case where siliconnitride oxide is used for the insulating layer 138 and silicon oxide isused for the insulating layer 140, the insulating layer 138 functions asan etching stopper.

Next, the insulating layer 138 and the insulating layer 140 aresubjected to CMP treatment, or etching treatment, so that upper surfacesof the gate electrode 128 a and the conductive layer 128 b are exposed(see FIG. 8D). Here, etching is performed until the gate electrode 128 aand the conductive layer 128 b are partly exposed. For the etchingtreatment, dry etching is preferably performed, but wet etching may beperformed. In the step of partly exposing the gate electrode 128 a andthe conductive layer 128 b, in order to improve the characteristics ofthe transistor 162 which is formed later, the surfaces of the insulatinglayer 136, the insulating layer 138, and the insulating layer 140 arepreferably flattened as much as possible.

Through the above steps, the transistor 160 in the lower portion can beformed (see FIG. 8D).

Note that before or after the above steps, a step for forming anadditional electrode, wiring, semiconductor layer, or insulating layermay be performed. For example, a multilayer wiring structure in which aninsulating layer and a conductive layer are stacked is employed as awiring structure, so that a highly-integrated semiconductor device canbe provided.

<Manufacturing Method of Transistor in Upper Portion>

Next, a method for manufacturing the transistor 162 in the upper portionwill be described with reference to FIGS. 9A to 9D and FIGS. 10A and10C.

First, an oxide semiconductor layer is formed over the gate electrode128 a, the conductive layer 128 b, the insulating layer 136, theinsulating layer 138, the insulating layer 140, and the like and isprocessed, so that the oxide semiconductor layer 144 is formed (see FIG.9A). Note that an insulating layer functioning as a base may be formedover the insulating layer 136, the insulating layer 138, and theinsulating layer 140 before the oxide semiconductor layer is formed. Theinsulating layer can be formed by a PVD method such as a sputteringmethod, or a CVD method such as a plasma CVD method.

An oxide semiconductor to be used preferably contains at least indium(In) or zinc (Zn). In particular, In and Zn are preferably contained. Asa stabilizer for reducing change in electrical characteristics of atransistor using the oxide semiconductor, gallium (Ga) is preferablyadditionally contained. Tin (Sn) is preferably contained as astabilizer. Hafnium (Hf) is preferably contained as a stabilizer.Aluminum (Al) is preferably contained as a stabilizer.

As another stabilizer, one or plural kinds of lanthanoid such aslanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium(Lu) may be contained.

As the oxide semiconductor, for example, an indium oxide, a tin oxide, azinc oxide, a two-component metal oxide such as an In—Zn-based oxide, aSn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, aSn—Mg-based oxide, an In—Mg-based oxide, or an In—Ga-based oxide, athree-component metal oxide such as an In—Ga—Zn-based oxide (alsoreferred to as IGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide,a Sn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide,an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-basedoxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, anIn—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide,an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-basedoxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, anIn—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide, or a four-componentmetal oxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-basedoxide, an In—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide can be used.

Note that here, for example, an “In—Ga—Zn—O-based oxide” means an oxidecontaining In, Ga, and Zn as its main component and there is noparticular limitation on the ratio of In:Ga:Zn. Further, a metal elementin addition to In, Ga, and Zn may be contained.

As a material used for the oxide semiconductor layer, a four-componentmetal oxide such as an In—Sn—Ga—Zn—O-based material; a three-componentmetal oxide such as an In—Ga—Zn—O-based material, an In—Sn—Zn—O-basedmaterial, an In—Al—Zn—O-based material, a Sn—Ga—Zn—O-based material, anAl—Ga—Zn—O-based material, or a Sn—Al—Zn—O-based material; atwo-component metal oxide such as an In—Zn—O-based material, aSn—Zn—O-based material, an Al—Zn—O-based material, a Zn—Mg—O-basedmaterial, a Sn—Mg—O-based material, an In—Mg—O-based material, or anIn—Ga—O-based material; or a single-component metal oxide such as anIn—O-based material; a Sn—O-based material; a Zn—O-based material; orthe like can be used. In addition, the above materials may contain SiO₂.Here, for example, an In—Ga—Zn—O-based material means an oxide filmcontaining indium (In), gallium (Ga), and zinc (Zn), and there is noparticular limitation on the composition ratio. Further, theIn—Ga—Zn—O-based oxide semiconductor may contain an element other thanIn, Ga, and Zn.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=1:1:1 (=1/3:1/3:1/3) or In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), or anyof oxides whose composition is in the neighborhood of the abovecompositions can be used. Alternatively, an In—Sn—Zn-based oxide with anatomic ratio of In:Sn:Zn=1:1:1 (=1/3:1/3:1/3), In:Sn:Zn=2:1:3(=1/3:1/6:1/2), or In:Sn:Zn=2:1:5 (=1/4:1/8:5/8), or any of oxides whosecomposition is in the neighborhood of the above compositions may beused.

However, without limitation to the materials given above, a materialwith an appropriate composition may be used depending on neededsemiconductor characteristics (e.g., mobility, threshold voltage, andvariation). In order to obtain the needed semiconductor characteristics,it is preferable that the carrier density, the impurity concentration,the defect density, the atomic ratio between a metal element and oxygen,the interatomic distance, the density, and the like be set toappropriate values.

For example, high mobility can be obtained relatively easily in the caseof using an In—Sn—Zn-based oxide. However, mobility can be increased byreducing the defect density in a bulk also in the case of using anIn—Ga—Zn-based oxide.

Note that for example, the expression “the composition of an oxideincluding In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1),is in the neighborhood of the composition of an oxide including In, Ga,and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1)” means that a, b,and c satisfy the following relation: (a−A)²+(b−B)²+(c−C)²≦r², and r maybe 0.05, for example. The same applies to other oxides.

The oxide semiconductor may be either single crystal ornon-single-crystal. In the latter case, the oxide semiconductor may beeither amorphous or polycrystal. Further, the oxide semiconductor mayhave either an amorphous structure including a portion havingcrystallinity or a non-amorphous structure.

In an oxide semiconductor in an amorphous state, a flat surface can beobtained with relative ease, so that when a transistor is manufacturedwith the use of the oxide semiconductor, interface scattering can bereduced, and relatively high mobility can be obtained with relativeease.

In an oxide semiconductor having crystallinity, defects in the bulk canbe further reduced and when a surface flatness is improved, mobilityhigher than that of an oxide semiconductor layer in an amorphous statecan be obtained. In order to improve the surface flatness, the oxidesemiconductor is preferably formed over a flat surface. Specifically,the oxide semiconductor may be formed over a surface with the averagesurface roughness (Ra) of less than or equal to 1 nm, preferably lessthan or equal to 0.3 nm, more preferably less than or equal to 0.1 nm.

Note that, R_(a) is obtained by three-dimension expansion of center lineaverage roughness that is defined by JIS B0601 so as to be applied to aplane. The R_(a) can be expressed as an “average value of the absolutevalues of deviations from a reference surface to a specific surface” andis defined by the formula below.

$\begin{matrix}{{Ra} = {\frac{1}{S_{0}}{\int_{x_{2}}^{x_{1}}{\int_{y_{2}}^{y_{1}}{{{{f\left( {x,y} \right)} - Z_{0}}}\ d\; x\ d\; y}}}}} & \left\lbrack {{FORMULA}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In the above formula, S₀ represents an area of a plane to be measured (arectangular region which is defined by four points represented bycoordinates (x₁, y₁), (x₁, y₂), (x₂, y₁), and (x₂, y₂)), and Z₀represents an average height of the plane to be measured. R_(a) can bemeasured with an atomic force microscope (AFM).

As the oxide semiconductor layer, a thin film including a materialexpressed as the chemical formula, InMO₃(ZnO)_(m) (m>0) can be used.Here, M represents one or more metal elements selected from Ga, Al, Mn,and Co. For example, M may be Ga, Ga and Al, Ga and Mn, Ga and Co, orthe like.

The thickness of the oxide semiconductor layer is preferably greaterthan or equal to 3 nm and less than or equal to 30 nm for the followingreason: the transistor might possibly be normally on when the oxidesemiconductor layer is too thick (e.g., a thickness of 50 nm or more).

The oxide semiconductor layer is preferably formed by a method in whichimpurities such as hydrogen, water, a hydroxyl group, or hydride do notenter the oxide semiconductor layer. For example, a sputtering methodcan be used.

In the case where an In—Zn-based oxide material is used as an oxidesemiconductor, a target therefor has a composition ratio of In:Zn=50:1to 1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio),preferably, In:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2in a molar ratio), further preferably, In:Zn=15:1 to 1.5:1 in an atomicratio (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). For example, in a targetused for formation of an In—Zn-based oxide semiconductor which has anatomic ratio of In:Zn:O=X:Y:Z, the relation of Z>1.5X+Y is satisfied.

Further, an In—Sn—Zn oxide can be referred to as ITZO. An oxide targetwhich has an atomic ratio of In, Sn, and Zn of 1:2:2, 2:1:3, 1:1:1,20:45:35, or the like is used.

In this embodiment, the oxide semiconductor layer is formed by asputtering method with the use of an In—Ga—Zn-based oxide target.

As the In—Ga—Zn-based oxide target, for example, an oxide target havinga composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio] can be used.Note that it is not necessary to limit the material and the compositionratio of the target to the above. For example, an oxide target having acomposition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio] can be used.

The filling rate of the oxide target is greater than or equal to 90% andless than or equal to 100%, preferably greater than or equal to 95% andgreater than or equal to 99.9%. With the use of the metal oxide targetwith a high filling rate, a dense oxide semiconductor layer can beformed.

The deposition atmosphere may be a rare gas (typically argon)atmosphere, an oxygen atmosphere, or a mixed atmosphere containing arare gas and oxygen. An atmosphere of a high-purity gas from which animpurity such as hydrogen, water, a hydroxyl group, or hydride isremoved is preferable, in order to prevent hydrogen, water, a hydroxylgroup, hydride, or the like from entering the oxide semiconductor layer.

For example, the oxide semiconductor layer can be formed describedbelow.

First, the substrate is held in a deposition chamber which is kept underreduced pressure, and then is heated so that the substrate temperaturereaches a temperature higher than 200° C. and lower than or equal to500° C., preferably higher than 300° C. and lower than or equal to 500°C., further preferably higher than or equal to 350° C. and lower than orequal to 450° C.

Then, a high-purity gas in which impurities such as hydrogen, water, ahydroxyl group, or hydride are sufficiently removed is introduced intothe deposition chamber from which remaining moisture is being removed,and the oxide semiconductor layer is formed over the substrate with theuse of the target. In order to remove moisture remaining in thedeposition chamber, an entrapment vacuum pump such as a cryopump, an ionpump, or a titanium sublimation pump is desirably used. Further, anevacuation unit may be a turbo pump provided with a cold trap. In thedeposition chamber which is evacuated with the cryopump, for example,impurities such as hydrogen, water, a hydroxyl group, or hydride(preferably, also a compound containing a carbon atom) or the like areremoved, whereby the concentration of impurities such as hydrogen,water, a hydroxyl group, or hydride in the oxide semiconductor layerformed in the deposition chamber can be reduced.

In the case where the substrate temperature is low (for example, 100° C.or lower) during deposition, a substance including a hydrogen atom mayenter the oxide semiconductor; thus, it is preferable that the substratebe heated at a temperature in the above range. When the oxidesemiconductor layer is formed with the substrate heated at thetemperature, the substrate temperature is increased, so that hydrogenbonds are cut by heat and are less likely to be taken into the oxidesemiconductor layer. Therefore, the oxide semiconductor layer is formedwith the substrate heated at the above temperature, whereby theconcentration of impurities such as hydrogen, water, a hydroxyl group,or hydride in the oxide semiconductor layer can be sufficiently reduced.Moreover, damage due to sputtering can be reduced.

As an example of the film formation conditions, the following conditionscan be employed: the distance between the substrate and the target is 60mm, the pressure is 0.4 Pa, the direct-current (DC) power source is 0.5kW, the substrate temperature is 400° C., and the film formationatmosphere is an oxygen atmosphere (the proportion of the oxygen flowrate is 100%). Note that a pulse direct current power source ispreferable because powder substances (also referred to as particles ordust) generated in deposition can be reduced and the film thickness canbe uniform.

Note that before the oxide semiconductor layer is formed by a sputteringmethod, powdery substances (also referred to as particles or dust)attached on a formation surface of the oxide semiconductor layer arepreferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering refers to amethod in which voltage is applied to a substrate side to generateplasma in the vicinity of the substrate to modify a surface. Note that agas of nitrogen, helium, oxygen or the like may be used instead ofargon.

The oxide semiconductor layer can be processed by being etched after amask having a desired shape is formed over the oxide semiconductorlayer. The mask may be formed by a method such as photolithography or anink-jet method. The metal oxide film and the like are also processedwhile the oxide semiconductor film is processed. Note that the etchingof the oxide semiconductor layer may be dry etching or wet etching.Needless to say, both of them may be employed in combination.

After that, heat treatment (first heat treatment) may be performed onthe oxide semiconductor layer 144. The heat treatment further removessubstances including hydrogen atoms in the oxide semiconductor layer144. The heat treatment is performed under an inert gas atmosphere atgreater than or equal to 250° C. and less than or equal to 700° C.,preferably greater than or equal to 450° C. and less than or equal to600° C. or less than a strain point of the substrate. The inert gasatmosphere is preferably an atmosphere which contains nitrogen or a raregas (e.g., helium, neon, or argon) as its main component and does notcontain water, hydrogen, or the like. For example, the purity ofnitrogen or a rare gas such as helium, neon, or argon introduced into aheat treatment apparatus is greater than or equal to 6 N (99.9999%),preferably greater than or equal to 7 N (99.99999%) (that is, theconcentration of the impurities is less than or equal to 1 ppm,preferably less than or equal to 0.1 ppm).

The heat treatment can be performed in such a manner that, for example,an object to be heated is introduced into an electric furnace in which aresistance heating element or the like is used and heated, under anitrogen atmosphere at 450° C. for an hour. The oxide semiconductorlayer 144 is not exposed to the air during the heat treatment so thatentry of water and hydrogen can be prevented.

The above heat treatment has an effect of removing hydrogen, water, andthe like and can be referred to as dehydration treatment,dehydrogenation treatment, or the like. The heat treatment can beperformed at the timing, for example, before the oxide semiconductorlayer is processed into an island shape or after the gate insulatinglayer is formed. Such dehydration treatment or dehydrogenation treatmentmay be performed once or plural times.

Next, a conductive layer for forming a source electrode and a drainelectrode (including a wiring formed in the same layer as the sourceelectrode and the drain electrode) is formed over the oxidesemiconductor layer 144 and the like and is processed, so that thesource and drain electrodes 142 a and 142 b are formed (see FIG. 9B).

The conductive layer can be formed by a PVD method or a CVD method. As amaterial for the conductive layer, an element selected from aluminum,chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloycontaining any of these elements as a component; or the like can beused. Further, one or more materials selected from manganese, magnesium,zirconium, beryllium, neodymium, and scandium may be used.

The conductive layer may have a single-layer structure or a stackedstructure including two or more layers. For example, the conductivelayer may have a single-layer structure of a titanium film or a titaniumnitride film, a single-layer structure of an aluminum film containingsilicon, a two-layer structure in which a titanium film is stacked overan aluminum film, a two-layer structure in which a titanium film isstacked over a titanium nitride film, or a three-layer structure inwhich a titanium film, an aluminum film, and a titanium film are stackedin this order. Note that the conductive layer having a single-layerstructure of a titanium film or a titanium nitride film has an advantagein that it can be easily processed into the source electrode 142 a andthe drain electrode 142 b having tapered shapes.

The conductive layer may be formed using a conductive metal oxide. Asthe conductive metal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zincoxide (ZnO), an indium oxide-tin oxide compound (In₂O₃—SnO₂, which isabbreviated to ITO in some cases), an indium oxide-zinc oxide compound(In₂O₃—ZnO), or any of these metal oxide materials in which silicon orsilicon oxide is contained can be used.

The conductive layer is preferably etched so that end portions of thesource electrode 142 a and the drain electrode 142 b that are to beformed are tapered. Here, a taper angle is, for example, preferablygreater than or equal to 30° and less than or equal to 60°. The etchingis performed so that the end portions of the source electrode 142 a andthe drain electrode 142 b are tapered, whereby coverage with the gateinsulating layer 146 formed later can be improved and disconnection canbe prevented.

The channel length (L) of the transistor in the upper portion isdetermined by a distance between lower edge portions of the sourceelectrode 142 a and the drain electrode 142 b. Note that for lightexposure for forming a mask used in the case where a transistor with achannel length (L) of less than 25 nm is formed, it is preferable to useextreme ultraviolet rays whose wavelength is as short as severalnanometers to several tens of nanometers. In the light exposure byextreme ultraviolet light, the resolution is high and the focus depth islarge. For these reasons, the channel length (L) of the transistor to beformed later can be in the range of greater than or equal to 10 nm andless than or equal to 1000 nm (1 μm), and the circuit can operate athigher speed. Moreover, miniaturization allows low power consumption ofa semiconductor device.

As an example which is different from that in FIG. 9B, oxide conductivelayers can be provided as a source region and a drain region, betweenthe oxide semiconductor layer 144 and the source and drain electrodes. Amaterial of the oxide conductive layer preferably contains zinc oxide asa component and preferably does not contain indium oxide. For such anoxide conductive layer, zinc oxide, aluminum zinc oxide, aluminum zincoxynitride, gallium zinc oxide, or the like can be used.

For example, the oxide conductive layers which serve as a source regionand a drain region, the source electrode 142 a, and the drain electrode142 b can be formed by forming an oxide conductive film over the oxidesemiconductor layer 144, forming a conductive layer over the oxideconductive film, and processing the oxide conductive film and theconductive layer in one photolithography step.

Alternatively, a stacked layer of an oxide semiconductor film and anoxide conductive film is formed and the stacked layer is processed inone photolithography step, so that the island-shaped oxide semiconductorlayer 144 and oxide conductive film may be formed. After the sourceelectrode 142 a and the drain electrode 142 b are formed, theisland-shaped oxide conductive film is etched using the source electrode142 a and the drain electrode 142 b as masks, so that the oxideconductive layers which serve as a source region and a drain region canbe formed.

Note that when etching treatment for processing the oxide conductivelayer is performed, etching conditions (e.g., type of etching agent, theconcentration of an etching agent, and etching time) are adjusted asappropriate in order to prevent excessive etching of the oxidesemiconductor layer.

When the oxide conductive layer is provided between the oxidesemiconductor layer and the source and drain electrode layers, thesource region and the drain region can have lower resistance and thetransistor can operate at high speed. Moreover, with the structureincluding the oxide semiconductor layer 144, the oxide conductive layer,and the drain electrode formed using a metal material, withstand voltageof the transistor can be further increased.

In order to improve frequency characteristics of a peripheral circuit (adriver circuit), it is effective to use the oxide conductive layer for asource region and a drain region for the reason below. The contactresistance can be reduced when a metal electrode (e.g., molybdenum ortungsten) and the oxide conductive layer are in contact, as compared tothe case where a metal electrode (e.g., molybdenum or tungsten) and theoxide semiconductor layer are in contact. The contact resistance can bereduced by interposing an oxide conductive layer between the oxidesemiconductor layer and source and drain electrodes; thus, frequencycharacteristics of a peripheral circuit (driver circuit) can beimproved.

Next, the gate insulating layer 146 is formed so as to cover the sourceelectrode 142 a and the drain electrode 142 b and to be in contact withpart of the oxide semiconductor layer 144 (see FIG. 9C).

The gate insulating layer 146 can be formed by a CVD method, asputtering method, or the like. The gate insulating layer 146 ispreferably formed so as to contain silicon oxide, silicon nitride,silicon oxynitride, aluminum oxide, tantalum oxide, hafnium oxide,yttrium oxide, gallium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0,y>0)), hafnium silicate to which nitrogen is added (HfSi_(x)O_(y)N_(z)(x>0, y>0, z>0)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z) (x>0, y>0, z>0)), or the like. The gate insulatinglayer 146 may have a single-layer structure or a stacked structure inwhich these elements are combined. There is no particular limitation onthe thickness; however, in the case where a semiconductor device isminiaturized, the thickness is preferably small for ensuring operationof the transistor. For example, in the case where silicon oxide is used,the thickness can be set to greater than or equal to 1 nm and less thanor equal to 100 nm, preferably greater than or equal to 10 nm and lessthan or equal to 50 nm.

When the gate insulating layer is thin as described above, a problem ofgate leakage due to a tunnel effect or the like is caused. In order tosolve the problem of gate leakage, a high dielectric constant (high-k)material such as hafnium oxide, tantalum oxide, yttrium oxide, hafniumsilicate (HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate to which nitrogenis added (HfSi_(x)O_(y)N_(z) (x>0, y>0, z>0)), or hafnium aluminate towhich nitrogen is added (HfAl_(x)O_(y)N_(z) (x>0, y>0, z>0)) ispreferably used for the gate insulating layer 146. The use of a high-kmaterial for the gate insulating layer 146 makes it possible to ensureelectrical characteristics and to increase the thickness in order tosuppress gate leakage. Note that a stacked structure of a filmcontaining a high-k material and a film containing any one of siliconoxide, silicon nitride, silicon oxynitride, silicon nitride oxide,aluminum oxide, and the like may be employed.

Further, the insulating layer in contact with the oxide semiconductorlayer 144 (in this embodiment, the gate insulating layer 146) may be aninsulating material containing an element belonging to Group 13 andoxygen. Many oxide semiconductor materials contain an element belongingto Group 13, and an insulating material containing an element belongingto Group 13 is compatible with an oxide semiconductor. By using such aninsulating material for the insulating layer in contact with the oxidesemiconductor layer, an interface with the oxide semiconductor layer canbe kept favorable.

An insulating material containing an element belonging to Group 13refers to an insulating material containing one or more elementsbelonging to Group 13. As examples of the insulating material containingan element belonging to Group 13, a gallium oxide, an aluminum oxide, analuminum gallium oxide, a gallium aluminum oxide, and the like aregiven. Here, aluminum gallium oxide refers to a material in which theamount of aluminum is larger than that of gallium in atomic percent, andgallium aluminum oxide refers to a material in which the amount ofgallium is larger than or equal to that of aluminum in atomic percent.

For example, in the case of forming a gate insulating layer in contactwith an oxide semiconductor layer containing gallium, the use of amaterial containing gallium oxide for the gate insulating layer allowsthe characteristics of the interface between the oxide semiconductorlayer and the gate insulating layer to be kept favorable. Moreover, whenthe oxide semiconductor layer and the insulating layer containinggallium oxide are provided in contact with each other, pileup ofhydrogen at the interface between the oxide semiconductor layer and theinsulating layer can be reduced. Note that a similar effect can beobtained in the case where an element belonging to the same group as aconstituent element of the oxide semiconductor is used for theinsulating layer. For example, it is effective to form an insulatinglayer with the use of a material containing an aluminum oxide. Aluminumoxide is impermeable to water. Therefore, it is preferable to use amaterial containing aluminum oxide in terms of preventing entry of waterto the oxide semiconductor layer.

The insulating layer in contact with the oxide semiconductor layer 144preferably contains oxygen in a proportion higher than that in thestoichiometric composition, by heat treatment under an oxygen atmosphereor oxygen doping. “Oxygen doping” refers to addition of oxygen into abulk. Note that the term “bulk” is used in order to clarify that oxygenis added not only to a surface of a thin layer but also to the inside ofthe thin layer. In addition, “oxygen doping” includes “oxygen plasmadoping” in which oxygen which is made to be plasma is added to a bulk.The oxygen doping may be performed using an ion implantation method oran ion doping method.

For example, in the case where the insulating layer in contact with theoxide semiconductor layer 144 is formed using gallium oxide, thecomposition of gallium oxide can be set to be Ga₂O_(x) (x=3+α, 0<α<1) byheat treatment under an oxygen atmosphere or oxygen doping. Further, inthe case where the insulating layer in contact with the oxidesemiconductor layer 144 is formed of aluminum oxide, the composition ofaluminum oxide can be set to be Al₂O_(x) (x=3+α, 0<α<1) by heattreatment under an oxygen atmosphere or oxygen doping. Further, in thecase where the insulating layer in contact with the oxide semiconductorlayer 144 is formed of gallium aluminum oxide (or aluminum galliumoxide), the composition of gallium aluminum oxide (or aluminum galliumoxide) can be set to be Ga_(x)Al_(2-x)O_(3+α) (0<x<2, 0<α<1) by heattreatment under an oxygen atmosphere or oxygen doping.

By oxygen doping or the like, an insulating layer which includes aregion where the proportion of oxygen is higher than that in thestoichiometric composition can be formed. When the insulating layerhaving such a region is in contact with the oxide semiconductor layer,oxygen that exists excessively in the insulating layer is supplied tothe oxide semiconductor layer, which allows oxygen deficiency in theoxide semiconductor layer or at an interface between the oxidesemiconductor layer and the insulating layer to be reduced.

The insulating layer which includes a region where the proportion ofoxygen is higher than that in the stoichiometric composition may beapplied to the insulating layer which serves as a base film of the oxidesemiconductor layer 144, instead of the gate insulating layer 146, ormay be applied to both the gate insulating layer 146 and the base film.

After the gate insulating layer 146 is formed, second heat treatment ispreferably performed in an inert gas atmosphere or an oxygen atmosphere.The temperature of the heat treatment is higher than or equal to 200° C.and lower than or equal to 450° C., preferably higher than or equal to250° C. and lower than or equal to 350° C. For example, the heattreatment may be performed at 250° C. for an hour in a nitrogenatmosphere. The second heat treatment allows a reduction in variation inthe electrical characteristics of the transistor. Further, in the casewhere the gate insulating layer 146 contains oxygen, oxygen is suppliedto the oxide semiconductor layer 144 which has been subjected to thedehydration treatment or dehydrogenation treatment to compensate foroxygen deficiency in the oxide semiconductor layer 144, so that ani-type (intrinsic) or substantially i-type oxide semiconductor layer canbe formed.

Note that although the second heat treatment is performed after the gateinsulating layer 146 is formed in this embodiment, the timing of thesecond heat treatment is not limited thereto. The second heat treatmentmay be performed, for example, after the gate electrode is formed.Alternatively, the second heat treatment may be performed following thefirst heat treatment, the first heat treatment may double as the secondheat treatment, or the second heat treatment may double as the firstheat treatment.

Next, a conductive layer for forming a gate electrode (including awiring formed using the same layer as the gate electrode) is formed andis processed, so that the gate electrode 148 a and the conductive layer148 b are formed (see FIG. 9D).

The gate electrode 148 a and the conductive layer 148 b can be formedusing a metal material such as molybdenum, titanium, tantalum, tungsten,aluminum, copper, neodymium, or scandium, or an alloy materialcontaining any of these materials as a main component. Note that thegate electrode 148 a and the conductive layer 148 b may have asingle-layer structure or a stacked structure.

Next, an insulating layer 150 is formed over the gate insulating layer146, the gate electrode 148 a, and the conductive layer 148 b (see FIG.10A). The insulating layer 150 can be formed by a PVD method, a CVDmethod, or the like. The insulating layer 150 can be formed using amaterial containing an inorganic insulating material such as siliconoxide, silicon oxynitride, silicon nitride, hafnium oxide, galliumoxide, or aluminum oxide. Note that for the insulating layer 150, amaterial with a low dielectric constant may be preferably used or astructure with a low dielectric constant (e.g., a porous structure) ispreferably employed for the following reason: the low dielectricconstant of the insulating layer 150 allows capacitance generatedbetween wirings, electrodes, or the like to be reduced and operationspeed to be increased. Note that although the insulating layer 150 has asingle-layer structure in this embodiment, an embodiment of thedisclosed invention is not limited to this. The insulating layer 150 mayhave a stacked structure including two or more layers.

Next, an opening reaching the source electrode 142 a is formed in thegate insulating layer 146 and the insulating layer 150. After that, awiring 154 connected to the source electrode 142 a is formed over theinsulating layer 150 (see FIG. 10B). The opening is formed by selectiveetching using a mask or the like.

A conductive layer is formed by a PVD method or a CVD method and then ispatterned, so that the wiring 154 is formed. As a material for theconductive layer, an element selected from aluminum, chromium, copper,tantalum, titanium, molybdenum, and tungsten; an alloy containing any ofthese elements as a component; or the like can be used. Further, one ormore materials selected from manganese, magnesium, zirconium, beryllium,neodymium, and scandium may be used.

Specifically, it is possible to employ a method, for example, in which athin (about 5 nm) titanium film is formed in a region including theopening of the insulating layer 150 by a PVD method, and then, analuminum film is formed so as to be embedded in the openings. Here, thetitanium film formed by a PVD method has a function of reducing an oxidefilm (such as a natural oxide film) over which the titanium film is tobe formed, and thereby lowering contact resistance with the lowerelectrode or the like (here, the source electrode 142 a). In addition,hillock of aluminum film can be prevented. A copper film may be formedby a plating method after the formation of the barrier film of titanium,titanium nitride, or the like.

The opening formed in the insulating layer 150 is preferably formed in aregion overlapping with the conductive layer 128 b. By forming theopening in such a region, an increase in the element area due to acontact region of electrodes can be suppressed.

Here, the case where the position where the impurity region 126 and thesource electrode 142 a are connected and the position where the sourceelectrode 142 a and the wiring 154 are connected overlap with each otherwithout using the conductive layer 128 b will be described. In thiscase, an opening (also referred to as a contact in a lower portion) isformed in the insulating layer 136, the insulating layer 138, and theinsulating layer 140 which are formed over the impurity region 126, andthe source electrode 142 a is formed in the contact in the lowerportion; after that, an opening (also referred to as a contact in anupper portion) is formed in a region overlapping with the contact in thelower portion in the gate insulating layer 146 and the insulating layer150, and then the wiring 154 is formed. When the contact in the upperportion is formed in the region overlapping with the contact in thelower portion, the source electrode 142 a formed in the contact in thelower portion might be disconnected due to etching. When the contacts inthe lower portion and in the upper portion are formed so as not tooverlap with each other in order to avoid the disconnection, an increasein the element area is caused.

As described in this embodiment, with the use of the conductive layer128 b, the contact in the upper portion can be formed withoutdisconnection of the source electrode 142 a. Thus, the contacts in thelower portion and in the upper portion can be formed so as to overlapwith each other, so that the increase in the element area due to thecontact regions can be suppressed. In other words, the degree ofintegration of the semiconductor device can be increased.

Next, an insulating layer 156 is formed so as to cover the wiring 154(see FIG. 10C).

Through the above steps, the transistor 162 and the capacitor 164including the purified oxide semiconductor layer 144 are completed (seeFIG. 10C).

Since the oxide semiconductor layer 144 is purified in the transistor162 described in this embodiment, the hydrogen concentration is 5×10¹⁹atoms/cm³ or lower, preferably 5×10¹⁸ atoms/cm³ or lower, morepreferably 5×10¹⁷ atoms/cm³ or lower. In addition, the value of thecarrier concentration of the oxide semiconductor layer 144 issufficiently low (e.g., lower than 1×10¹²/cm³, preferably lower than1.45×10¹⁰/cm³) in comparison with that of a general silicon wafer(approximately 1×10¹⁴/cm³). Accordingly, the off-state current is alsosufficiently small. For example, the off-state current (here, currentper unit channel width (1 μm)) at room temperature (25° C.) is 100 zA (1zA (zeptoampere) is 1×10⁻²¹ A) or less, preferably 10 zA or less.

In this manner, by using the purified intrinsic oxide semiconductorlayer 144, the off-state current of the transistor can be sufficientlyreduced easily. In addition, by using such a transistor, a semiconductordevice in which stored data can be held for an extremely long time canbe obtained.

Further, in the semiconductor device described in this embodiment, awiring can be shared; thus, a semiconductor device with sufficientlyincreased degree of integration can be achieved.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

Embodiment 3

In this embodiment, the case where the semiconductor device described inthe above embodiments is applied to electronic devices will be describedwith reference to FIGS. 11A to 11F. In this embodiment, examples of theelectronic device to which the semiconductor device described in any ofthe above embodiments is applied include a computer, a mobile phone(also referred to as a cellular phone or a mobile phone device), apersonal digital assistant (including a portable game machine, an audioreproducing device, and the like), a camera such as a digital camera ora digital video camera, an electronic paper, and a television device(also referred to as a television or a television receiver).

FIG. 11A illustrates a laptop personal computer, which includes ahousing 701, a housing 702, a display portion 703, a keyboard 704, andthe like. The semiconductor device described in any of the aboveembodiments is provided in at least one of the housing 701 and thehousing 702. Therefore, a laptop personal computer in which writing andreading of data are performed at high speed, data is stored for a longtime, and power consumption is sufficiently reduced can be realized.

FIG. 11B illustrates a personal digital assistant (PDA). A main body 711is provided with a display portion 713, an external interface 715,operation buttons 714, and the like. Further, a stylus 712 and the likefor operation of the personal digital assistant are provided. In themain body 711, the semiconductor device described in any of the aboveembodiments is provided. Therefore, a personal digital assistant inwhich writing and reading of data are performed at high speed, data isstored for a long time, and power consumption is sufficiently reducedcan be realized.

FIG. 11C illustrates an e-book reader 720 incorporating an electronicpaper, which includes a housing 721 and a housing 723. The housing 721and the housing 723 are provided with a display portion 725 and adisplay portion 727, respectively. The housing 721 and the housing 723are connected by a hinge portion 737 and can be opened or closed withthe hinge portion 737. The housing 721 is provided with a power supply731, an operation key 733, a speaker 735, and the like. At least one ofthe housing 721 and the housing 723 is provided with the semiconductordevice described in any of the above embodiments. Therefore, an e-bookreader in which writing and reading of data are performed at high speed,data is stored for a long time, and power consumption is sufficientlyreduced can be realized.

FIG. 11D illustrates a mobile phone, which includes a housing 740 and ahousing 741. Moreover, the housing 740 and the housing 741 in a statewhere they are developed as illustrated in FIG. 11D can be slid so thatone is lapped over the other. Therefore, the size of the mobile phonecan be reduced, which makes the mobile phone suitable for being carriedaround. The housing 741 includes a display panel 742, a speaker 743, amicrophone 744, an operation key 745, a pointing device 746, a cameralens 747, an external connection terminal 748, and the like. The housing740 includes a solar cell 749 for charging the cellular phone, anexternal memory slot 750, and the like. In addition, an antenna isincorporated in the housing 741. At least one of the housing 740 and thehousing 741 is provided with the semiconductor device described in anyof the above embodiments. Therefore, a mobile phone in which writing andreading of data are performed at high speed, data is stored for a longtime, and power consumption is sufficiently reduced can be realized.

FIG. 11E illustrates a digital camera, which includes a main body 761, adisplay portion 767, an eyepiece 763, an operation switch 764, a displayportion 765, a battery 766, and the like. In the main body 761, thesemiconductor device described in any of the above embodiments isprovided. Therefore, a digital camera in which writing and reading ofdata are performed at high speed, data is stored for a long time, andpower consumption is sufficiently reduced can be realized.

FIG. 11F illustrates a television device 770, which includes a housing771, a display portion 773, a stand 775, and the like. The televisiondevice 770 can be operated with an operation switch of the housing 771or a remote controller 780. The semiconductor device described in any ofthe above embodiments is mounted in the housing 771 and the remotecontroller 780. Therefore, a television device in which writing andreading of data are performed at high speed, data is stored for a longtime, and power consumption is sufficiently reduced can be realized.

As described above, the electronic devices described in this embodimenteach include the semiconductor device according to any of the aboveembodiments. Therefore, electronic devices with low power consumptioncan be realized.

Embodiment 4

In this embodiment, the transistor including an oxide semiconductor as asemiconductor material, which has been described in Embodiments 1 to 3,will be described in detail. Specifically, as the oxide semiconductor,an oxide including a crystal with c-axis alignment (also referred to asC-Axis Aligned Crystal (CAAC)), which has a triangular or hexagonalatomic arrangement when seen from the direction of an a-b plane, asurface, or an interface will be described. In the crystal, metal atomsare arranged in a layered manner, or metal atoms and oxygen atoms arearranged in a layered manner along the c-axis, and the direction of thea-axis or the b-axis is varied in the a-b plane (the crystal rotatesaround the c-axis).

In a broad sense, an oxide including CAAC means a non-single-crystaloxide including a phase which has a triangular, hexagonal, regulartriangular, or regular hexagonal atomic arrangement when seen from thedirection perpendicular to the a-b plane and in which metal atoms arearranged in a layered manner or metal atoms and oxygen atoms arearranged in a layered manner when seen from the direction perpendicularto the c-axis direction.

The CAAC is not a single crystal, but this does not mean that the CAACis composed of only an amorphous component. Although the CAAC includes acrystallized portion (crystalline portion), a boundary between onecrystalline portion and another crystalline portion is not clear in somecases.

In the case where oxygen is included in the CAAC, nitrogen may besubstituted for part of oxygen included in the CAAC. The c-axes ofindividual crystalline portions included in the CAAC may be aligned inone direction (e.g., a direction perpendicular to a surface of asubstrate over which the CAAC is formed or a surface of the CAAC).Alternatively, the normals of the a-b planes of the individualcrystalline portions included in the CAAC may be aligned in onedirection (e.g., a direction perpendicular to a surface of a substrateover which the CAAC oxide is formed or a surface of the CAAC).

The CAAC becomes a conductor, a semiconductor, or an insulator dependingon its composition or the like. The CAAC transmits or does not transmitvisible light depending on its composition or the like.

As an example of such a CAAC, there is a crystal which is formed into afilm shape and has a triangular or hexagonal atomic arrangement whenobserved from the direction perpendicular to a surface of the film or asurface of a supporting substrate, and in which metal atoms are arrangedin a layered manner or metal atoms and oxygen atoms (or nitrogen atoms)are arranged in a layered manner when a cross section of the film isobserved.

An example of a crystal structure of the CAAC will be described indetail with reference to FIGS. 12A to 12E, FIGS. 13A to 13C, and FIGS.14A to 14C. In FIGS. 12A to 12E, FIGS. 13A to 13C, and FIGS. 14A to 14C,the vertical direction corresponds to the c-axis direction and a planeperpendicular to the c-axis direction corresponds to the a-b plane,unless otherwise specified. When the expressions “an upper half” and “alower half” are simply used, they refer to an upper half above the a-bplane and a lower half below the a-b plane (an upper half and a lowerhalf with respect to the a-b plane).

FIG. 12A illustrates a structure including one hexacoordinate In atomand six tetracoordinate oxygen (hereinafter referred to astetracoordinate O) atoms proximate to the In atom. Here, a structureincluding one metal atom and oxygen atoms proximate thereto is referredto as a small group. The structure in FIG. 12A is actually an octahedralstructure, but is illustrated as a planar structure for simplicity. Notethat three tetracoordinate O atoms exist in each of an upper half and alower half in FIG. 12A. In the small group illustrated in FIG. 12A,electric charge is 0.

FIG. 12B illustrates a structure including one pentacoordinate Ga atom,three tricoordinate oxygen (hereinafter referred to as tricoordinate O)atoms proximate to the Ga atom, and two tetracoordinate O atomsproximate to the Ga atom. All the tricoordinate O atoms exist on the a-bplane. One tetracoordinate O atom exists in each of an upper half and alower half in FIG. 12B. An In atom can also have the structureillustrated in FIG. 12B because an In atom can have five ligands. In thesmall group illustrated in FIG. 12B, electric charge is 0.

FIG. 12C illustrates a structure including one tetracoordinate Zn atomand four tetracoordinate O atoms proximate to the Zn atom. In FIG. 12C,one tetracoordinate O atom exists in an upper half and threetetracoordinate O atoms exist in a lower half. Alternatively, threetetracoordinate O atoms may exist in the upper half and onetetracoordinate O atom may exist in the lower half in FIG. 12C. In thesmall group illustrated in FIG. 12C, electric charge is 0.

FIG. 12D illustrates a structure including one hexacoordinate Sn atomand six tetracoordinate O atoms proximate to the Sn atom. In FIG. 12D,three tetracoordinate O atoms exist in each of an upper half and a lowerhalf. In the small group illustrated in FIG. 12D, electric charge is +1.

FIG. 12E illustrates a small group including two Zn atoms. In FIG. 12E,one tetracoordinate O atom exists in each of an upper half and a lowerhalf. In the small group illustrated in FIG. 12E, electric charge is −1.

Here, a plurality of small groups form a medium group, and a pluralityof medium groups form a large group (also referred to as a unit cell).

Now, a rule of bonding between the small groups will be described. Thethree O atoms in the upper half with respect to the hexacoordinate Inatom in FIG. 12A each have three proximate In atoms in the downwarddirection, and the three O atoms in the lower half each have threeproximate In atoms in the upward direction. The one O atom in the upperhalf with respect to the pentacoordinate Ga atom has one proximate Gaatom in the downward direction, and the one O atom in the lower half hasone proximate Ga atom in the upward direction. The one O atom in theupper half with respect to the tetracoordinate Zn atom has one proximateZn atom in the downward direction, and the three O atoms in the lowerhalf each have three proximate Zn atoms in the upward direction. In thismanner, the number of the tetracoordinate O atoms above the metal atomis equal to the number of the metal atoms proximate to and below each ofthe tetracoordinate O atoms. Similarly, the number of thetetracoordinate O atoms below the metal atom is equal to the number ofthe metal atoms proximate to and above each of the tetracoordinate Oatoms. Since the coordination number of the tetracoordinate O atom is 4,the sum of the number of the metal atoms proximate to and below the Oatom and the number of the metal atoms proximate to and above the O atomis 4. Accordingly, when the sum of the number of tetracoordinate O atomsabove a metal atom and the number of tetracoordinate O atoms belowanother metal atom is 4, the two kinds of small groups including themetal atoms can be bonded. For example, in the case where thehexacoordinate metal (In or Sn) atom is bonded through threetetracoordinate O atoms in the lower half, it is bonded to thepentacoordinate metal (Ga or In) atom or the tetracoordinate metal (Zn)atom.

A metal atom whose coordination number is 4, 5, or 6 is bonded toanother metal atom through a tetracoordinate O atom in the c-axisdirection. In addition to the above, a medium group can be formed in adifferent manner by combining a plurality of small groups so that thetotal electric charge of the layered structure is 0.

FIG. 13A illustrates a model of a medium group included in a layeredstructure of an In—Sn—Zn—O-based material. FIG. 13B illustrates a largegroup including three medium groups. Note that FIG. 13C illustrates anatomic arrangement in the case where the layered structure in FIG. 13Bis observed from the c-axis direction.

In FIG. 13A, a tricoordinate O atom is omitted for simplicity, and atetracoordinate O atom is illustrated by a circle; the number in thecircle shows the number of tetracoordinate O atoms. For example, threetetracoordinate O atoms existing in each of an upper half and a lowerhalf with respect to a Sn atom is denoted by circled 3. Similarly, inFIG. 13A, one tetracoordinate O atom existing in each of an upper halfand a lower half with respect to an In atom is denoted by circled 1.FIG. 13A also illustrates a Zn atom proximate to one tetracoordinate Oatom in a lower half and three tetracoordinate O atoms in an upper half,and a Zn atom proximate to one tetracoordinate O atom in an upper halfand three tetracoordinate O atoms in a lower half.

In the medium group included in the layered structure of theIn—Sn—Zn—O-based material in FIG. 13A, in the order starting from thetop, a Sn atom proximate to three tetracoordinate O atoms in each of anupper half and a lower half is bonded to an In atom proximate to onetetracoordinate O atom in each of an upper half and a lower half, the Inatom is bonded to a Zn atom proximate to three tetracoordinate O atomsin an upper half, the Zn atom is bonded to an In atom proximate to threetetracoordinate O atoms in each of an upper half and a lower halfthrough one tetracoordinate O atom in a lower half with respect to theZn atom, the In atom is bonded to a small group that includes two Znatoms and is proximate to one tetracoordinate O atom in an upper half,and the small group is bonded to a Sn atom proximate to threetetracoordinate O atoms in each of an upper half and a lower halfthrough one tetracoordinate O atom in a lower half with respect to thesmall group. A plurality of such medium groups are bonded, so that alarge group is formed.

Here, electric charge for one bond of a tricoordinate O atom andelectric charge for one bond of a tetracoordinate O atom can be assumedto be −0.667 and −0.5, respectively. For example, electric charge of a(hexacoordinate or pentacoordinate) In atom, electric charge of a(tetracoordinate) Zn atom, and electric charge of a (pentacoordinate orhexacoordinate) Sn atom are +3, +2, and +4, respectively. Accordingly,electric charge in a small group including a Sn atom is +1. Therefore,electric charge of −1, which cancels +1, is needed to form a layeredstructure including a Sn atom. As a structure having electric charge of−1, the small group including two Zn atoms as illustrated in FIG. 12Ecan be given. For example, with one small group including two Zn atoms,electric charge of one small group including a Sn atom can be cancelled,so that the total electric charge of the layered structure can be 0.

When the large group illustrated in FIG. 13B is repeated, anIn—Sn—Zn—O-based crystal (In₂SnZn₃O₈) can be obtained. Note that alayered structure of the obtained In—Sn—Zn—O-based crystal can beexpressed as a composition formula, In₂SnZn₂O₇(ZnO)_(m) (m is 0 or anatural number).

The above-described rule also applies to the following oxides: afour-component metal oxide such as an In—Sn—Ga—Zn-based oxide; athree-component metal oxide such as an In—Ga—Zn-based oxide (alsoreferred to as IGZO), an In—Al—Zn-based oxide, a Sn—Ga—Zn-based oxide,an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-basedoxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, anIn—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide,an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-basedoxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, anIn—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide,or an In—Lu—Zn-based oxide; a two-component metal oxide such as anIn—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide, aZn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, or anIn—Ga-based oxide; and the like.

As an example, FIG. 14A illustrates a model of a medium group includedin a layered structure of an In—Ga—Zn—O-based material.

In the medium group included in the layered structure of theIn—Ga—Zn—O-based material in FIG. 14A, in the order starting from thetop, an In atom proximate to three tetracoordinate O atoms in each of anupper half and a lower half is bonded to a Zn atom proximate to onetetracoordinate O atom in an upper half, the Zn atom is bonded to a Gaatom proximate to one tetracoordinate O atom in each of an upper halfand a lower half through three tetracoordinate O atoms in a lower halfwith respect to the Zn atom, and the Ga atom is bonded to an In atomproximate to three tetracoordinate O atoms in each of an upper half anda lower half through one tetracoordinate O atom in a lower half withrespect to the Ga atom. A plurality of such medium groups are bonded, sothat a large group is formed.

FIG. 14B illustrates a large group including three medium groups. Notethat FIG. 14C illustrates an atomic arrangement in the case where thelayered structure in FIG. 14B is observed from the c-axis direction.

Here, since electric charge of a (hexacoordinate or pentacoordinate) Inatom, electric charge of a (tetracoordinate) Zn atom, and electriccharge of a (pentacoordinate) Ga atom are +3, +2, +3, respectively,electric charge of a small group including any of an In atom, a Zn atom,and a Ga atom is 0. As a result, the total electric charge of a mediumgroup having a combination of such small groups is always 0.

In order to form the layered structure of the In—Ga—Zn—O-based material,a large group can be formed using not only the medium group illustratedin FIG. 14A but also a medium group in which the arrangement of the Inatom, the Ga atom, and the Zn atom is different from that in FIG. 14A.

Embodiment 5

In this embodiment, mobility of the transistor including an oxidesemiconductor for a channel formation region, which has been describedin Embodiments 1 to 4 will be described.

The actually measured field-effect mobility of an insulated gatetransistor can be lower than its original mobility because of a varietyof reasons; this phenomenon occurs not only in the case of using anoxide semiconductor. One of the reasons that reduce the mobility is adefect inside a semiconductor or a defect at an interface between thesemiconductor and an insulating film. When a Levinson model is used, thefield-effect mobility on the assumption that no defect exists inside thesemiconductor can be calculated theoretically.

Assuming that the original mobility and the measured field-effectmobility of a semiconductor are μ₀ and μ, respectively, and a potentialbarrier (such as a grain boundary) exists in the semiconductor, themeasured field-effect mobility can be expressed as the followingformula.

$\begin{matrix}{\mu = {\mu_{0}{\exp\left( {- \frac{E}{kT}} \right)}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Here, E represents the height of the potential barrier, k represents theBoltzmann constant, and T represents the absolute temperature. When thepotential barrier is assumed to be attributed to a defect, the height ofthe potential barrier can be expressed as the following formulaaccording to the Levinson model.

$\begin{matrix}{E = {\frac{e^{2}N^{2}}{n} = \frac{e^{3}N^{2}t}{C_{ox}V_{g}}}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack\end{matrix}$

Here, e represents the elementary charge, N represents the averagedefect density per unit area in a channel formation region, ∈ representsthe permittivity of the semiconductor, n represents the number ofcarriers per unit area in the channel formation region, C_(ox)represents the capacitance per unit area, V_(g) represents the gatevoltage, and t represents the thickness of the channel formation region.In the case where the thickness of the semiconductor layer is less thanor equal to 30 nm, the thickness of the channel formation region may beregarded as being the same as the thickness of the semiconductor layer.The drain current I_(d) in a linear region can be expressed as thefollowing formula.

$\begin{matrix}{I_{d} = {\frac{W\;\mu\; V_{g}V_{d}C_{ox}}{L}{\exp\left( {- \frac{E}{kT}} \right)}}} & \left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack\end{matrix}$

Here, L represents the channel length and W represents the channelwidth, and L and W are each 10 μm. In addition, V_(d) represents thedrain voltage. When dividing both sides of the above equation by V_(g)and then taking logarithms of both sides, the following formula can beobtained.

$\begin{matrix}{{\ln\left( \frac{I_{d}}{V_{g}} \right)} = {{{\ln\left( \frac{W\;\mu\; V_{d}C_{ox}}{L} \right)} - \frac{E}{kT}} = {{\ln\left( \frac{W\;\mu\; V_{d}C_{ox}}{L} \right)} - \frac{e^{3}N^{2}t}{8\;{kT}\; ɛ\; C_{ox}V_{g}}}}} & \left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack\end{matrix}$

The right side of Formula 5 is a function of V_(g). From the formula, itis found that the defect density N can be obtained from the slope of aline in a graph which is obtained by plotting actual measured valueswith ln(I_(d)/V_(g)) as the ordinate and 1/V_(g) as the abscissa. Thatis, the defect density can be evaluated from the I_(d)−V_(g)characteristics of the transistor. The defect density N of an oxidesemiconductor in which the ratio of indium (In), tin (Sn), and zinc (Zn)is 1:1:1 is approximately 1×10¹²/cm².

On the basis of the defect density obtained in this manner, or the like,μ₀ can be calculated to be 120 cm²/Vs from Formula 2 and Formula 3. Themeasured mobility of an In—Sn—Zn oxide including a defect isapproximately 40 cm²/Vs. However, assuming that no defect exists insidethe semiconductor and at the interface between the semiconductor and aninsulating film, the mobility μ₀ of the oxide semiconductor is expectedto be 120 cm²/Vs.

Note that even when no defect exists inside a semiconductor, scatteringat an interface between a channel formation region and a gate insulatinglayer affects the transport property of the transistor. In other words,the mobility μ₁ at a position that is distance x away from the interfacebetween the channel formation region and the gate insulating layer canbe expressed as the following formula.

$\begin{matrix}{\frac{1}{\mu_{1}} = {\frac{1}{\mu_{0}} + {\frac{D}{B}{\exp\left( {- \frac{x}{l}} \right)}}}} & \left\lbrack {{Formula}\mspace{14mu} 6} \right\rbrack\end{matrix}$

Here, D represents the electric field in the gate direction, and B and lare constants. B and l can be obtained from actual measurement results;according to the above measurement results, B is 4.75×10⁷ cm/s and l is10 nm (the depth to which the influence of interface scatteringreaches). When D is increased (i.e., when the gate voltage isincreased), the second term of Formula 6 is increased and accordinglythe mobility μ₁ is decreased.

Calculation results of the mobility μ₂ of a transistor whose channelformation region includes an ideal oxide semiconductor without a defectinside the semiconductor are shown in FIG. 15. For the calculation,device simulation software Sentaurus Device manufactured by Synopsys,Inc. was used, and the bandgap, the electron affinity, the relativepermittivity, and the thickness of the oxide semiconductor were assumedto be 2.8 eV, 4.7 eV, 15, and 15 nm, respectively. These values wereobtained by measurement of a thin film that was formed by a sputteringmethod.

Further, the work functions of a gate electrode, a source electrode, anda drain electrode were assumed to be 5.5 eV, 4.6 eV, and 4.6 eV,respectively. The thickness of a gate insulating layer was assumed to be100 nm, and the relative permittivity thereof was assumed to be 4.1. Thechannel length and the channel width were each assumed to be 10 μm, andthe drain voltage V_(d) was assumed to be 0.1 V.

As shown in FIG. 15, the mobility has a peak of more than 100 cm²/Vs ata gate voltage that is a little over 1 V and is decreased as the gatevoltage becomes higher because the influence of interface scattering isincreased. Note that in order to reduce interface scattering, it isdesirable that a surface of the semiconductor layer be flat at theatomic level (atomic layer flatness).

Calculation results of characteristics of minute transistors which aremanufactured using an oxide semiconductor having such a mobility areshown in FIGS. 16A to 16C, FIGS. 17A to 17C, and FIGS. 18A to 18C. FIGS.19A and 19B illustrate cross-sectional structures of the transistorsused for the calculation. The transistors illustrated in FIGS. 19A and19B each include a semiconductor region 103 a and a semiconductor region103 c which have n⁺-type conductivity in an oxide semiconductor layer.The resistivities of the semiconductor region 103 a and thesemiconductor region 103 c are 2×10⁻³ Ωcm.

The transistor illustrated in FIG. 19A is formed over a base insulatingfilm 101 and an embedded insulator 102 which is embedded in the baseinsulating film 101 and formed of aluminum oxide. The transistorincludes the semiconductor region 103 a, the semiconductor region 103 c,an intrinsic semiconductor region 103 b serving as a channel formationregion therebetween, and a gate electrode 105. The width of the gateelectrode 105 is 33 nm.

A gate insulating layer 104 is formed between the gate electrode 105 andthe semiconductor region 103 b. In addition, a sidewall insulator 106 aand a sidewall insulator 106 b are formed on both side surfaces of thegate electrode 105, and an insulator 107 is formed over the gateelectrode 105 so as to prevent a short circuit between the gateelectrode 105 and another wiring. The sidewall insulator has a width of5 nm. A source electrode 108 a and a drain electrode 108 b are providedin contact with the semiconductor region 103 a and the semiconductorregion 103 c, respectively. Note that the channel width of thistransistor is 40 nm.

The transistor of FIG. 19B is the same as the transistor of FIG. 19A inthat it is formed over the base insulating film 101 and the embeddedinsulator 102 formed of aluminum oxide and that it includes thesemiconductor region 103 a, the semiconductor region 103 c, theintrinsic semiconductor region 103 b provided therebetween, the gateelectrode 105 having a width of 33 nm, the gate insulating layer 104,the sidewall insulator 106 a, the sidewall insulator 106 b, theinsulator 107, the source electrode 108 a, and the drain electrode 108b.

The transistor illustrated in FIG. 19A is different from the transistorillustrated in FIG. 19B in the conductivity type of semiconductorregions under the sidewall insulator 106 a and the sidewall insulator106 b. In the transistor illustrated in FIG. 19A, the semiconductorregions under the sidewall insulator 106 a and the sidewall insulator106 b are part of the semiconductor region 103 a having n⁺-typeconductivity and part of the semiconductor region 103 c having n⁺-typeconductivity, whereas in the transistor illustrated in FIG. 19B, thesemiconductor regions under the sidewall insulator 106 a and thesidewall insulator 106 b are part of the intrinsic semiconductor region103 b. In other words, in the semiconductor layer of FIG. 19B, a regionhaving a width of L_(off) in which the semiconductor region 103 a (thesemiconductor region 103 c) and the gate electrode 105 do not overlap isprovided. This region is called an offset region, and the width L_(off)is called an offset length. As is seen from the drawing, the offsetlength is equal to the width of the sidewall insulator 106 a (thesidewall insulator 106 b).

The other parameters used in calculation are as described above. For thecalculation, device simulation software Sentaurus Device manufactured bySynopsys, Inc. was used. FIGS. 16A to 16C show the gate electrodevoltage (V_(g): a potential difference between the gate electrode andthe source) dependence of the drain current (I_(d), a solid line) andthe mobility (μ, a dotted line) of the transistor having the structureillustrated in FIG. 19A. The drain current I_(d) is obtained bycalculation under the assumption that the drain voltage (a potentialdifference between the drain and the source) is +1 V and the mobility μis obtained by calculation under the assumption that the drain voltageis +0.1 V.

FIG. 16A shows the gate voltage dependence of the transistor in the casewhere the thickness of the gate insulating layer is 15 nm, FIG. 16Bshows that of the transistor in the case where the thickness of the gateinsulating layer is 10 nm, and FIG. 16C shows that of the transistor inthe case where the thickness of the gate insulating layer is 5 nm. Asthe gate insulating layer is thinner, the drain current I_(d) (off-statecurrent) particularly in an off state is significantly decreased. Incontrast, there is no noticeable change in the peak value of themobility μ and the drain current I_(d) in an on state (on-statecurrent). The graphs show that the drain current exceeds 10 μA, which isrequired in a transistor in a memory cell, and the like, at a gatevoltage of around 1 V.

FIGS. 17A to 17C show the gate voltage V_(g) dependence of the draincurrent I_(d) (a solid line) and the mobility μ (a dotted line) of thetransistor having the structure illustrated in FIG. 19B where the offsetlength L_(off) is 5 nm. The drain current I_(d) is obtained bycalculation under the assumption that the drain voltage is +1 V and themobility μ is obtained by calculation under the assumption that thedrain voltage is +0.1 V. FIG. 17A shows the gate voltage dependence ofthe transistor in the case where the thickness of the gate insulatinglayer is 15 nm, FIG. 17B shows that of the transistor in the case wherethe thickness of the gate insulating layer is 10 nm, and FIG. 17C showsthat of the transistor in the case where the thickness of the gateinsulating layer is 5 nm.

Further, FIGS. 18A to 18C show the gate voltage dependence of the draincurrent I_(d) (a solid line) and the mobility μ (a dotted line) of thetransistor having the structure illustrated in FIG. 19B where the offsetlength L_(off) is 15 nm. The drain current I_(d) is obtained bycalculation under the assumption that the drain voltage is +1 V and themobility μ is obtained by calculation under the assumption that thedrain voltage is +0.1 V. FIG. 18A shows the gate voltage dependence ofthe transistor in the case where the thickness of the gate insulatinglayer is 15 nm, FIG. 18B shows that of the transistor in the case wherethe thickness of the gate insulating layer is 10 nm, and FIG. 18C showsthat of the transistor in the case where the thickness of the gateinsulating layer is 5 nm.

In either of the structures, as the gate insulating layer is thinner,the off-state current is significantly decreased, whereas no noticeablechange arises in the peak value of the mobility μ and the on-statecurrent.

Note that the peak of the mobility μ is approximately 80 cm²/Vs in FIGS.16A to 16C, approximately 60 cm²/Vs in FIGS. 17A to 17C, andapproximately 40 cm²/Vs in FIGS. 18A to 18C; thus, the peak of themobility μ is decreased as the offset length L_(off) is increased.Further, the same applies to the off-state current. The on-state currentis also decreased as the offset length L_(off) is increased; however,the decrease in the on-state current is much more gradual than thedecrease in the off-state current. Further, the graphs show that ineither of the structures, the drain current exceeds 10 μA, which isrequired in a transistor in a memory cell, and the like, at a gatevoltage of around 1 V.

Embodiment 6

A transistor in which an oxide semiconductor including In, Sn, and Zn asmain components is used as a channel formation region, which has beendescribed in Embodiments 1 to 5 will be described, can have favorablecharacteristics by depositing the oxide semiconductor while heating asubstrate or by performing heat treatment after an oxide semiconductorfilm is formed. Note that a main component refers to an element includedin a composition at 5 atomic % or more.

By intentionally heating the substrate after formation of the oxidesemiconductor film including In, Sn, and Zn as main components, thefield-effect mobility of the transistor can be improved. Further, thethreshold voltage of the transistor can be positively shifted to makethe transistor normally off.

As an example, FIGS. 20A to 20C each show characteristics of atransistor in which an oxide semiconductor film including In, Sn, and Znas main components and having a channel length L of 3 μm and a channelwidth W of 10 μm, and a gate insulating layer with a thickness of 100 nmare used. Note that V_(d) was set to 10 V.

FIG. 20A shows characteristics of a transistor whose oxide semiconductorfilm including In, Sn, and Zn as main components was formed by asputtering method without heating a substrate intentionally. Thefield-effect mobility of the transistor is 18.8 cm²/Vsec. On the otherhand, when the oxide semiconductor film including In, Sn, and Zn as maincomponents is formed while heating the substrate intentionally, thefield-effect mobility can be improved. FIG. 20B shows characteristics ofa transistor whose oxide semiconductor film including In, Sn, and Zn asmain components was formed while heating a substrate at 200° C. Thefield-effect mobility of the transistor is 32.2 cm²/Vsec.

The field-effect mobility can be further improved by performing heattreatment after formation of the oxide semiconductor film including In,Sn, and Zn as main components. FIG. 20C shows characteristics of atransistor whose oxide semiconductor film including In, Sn, and Zn asmain components was formed by sputtering at 200° C. and then subjectedto heat treatment at 650° C. The field-effect mobility of the transistoris 34.5 cm²/Vsec.

The intentional heating of the substrate is expected to have an effectof reducing moisture taken into the oxide semiconductor film during theformation by sputtering. Further, the heat treatment after filmformation enables hydrogen, a hydroxyl group, or moisture to be releasedand removed from the oxide semiconductor film. In this manner, thefield-effect mobility can be improved. Such an improvement infield-effect mobility is presumed to be achieved not only by removal ofimpurities by dehydration or dehydrogenation but also by a reduction ininteratomic distance due to an increase in density. The oxidesemiconductor can be crystallized by being purified by removal ofimpurities from the oxide semiconductor. In the case of using such apurified non-single-crystal oxide semiconductor, ideally, a field-effectmobility exceeding 100 cm²/Vsec is expected to be realized.

The oxide semiconductor including In, Sn, and Zn as main components maybe crystallized in the following manner: oxygen ions are implanted intothe oxide semiconductor, hydrogen, a hydroxyl group, or moistureincluded in the oxide semiconductor is released by heat treatment, andthe oxide semiconductor is crystallized through the heat treatment or byanother heat treatment performed later. By such crystallizationtreatment or recrystallization treatment, a non-single-crystal oxidesemiconductor having favorable crystallinity can be obtained.

The intentional heating of the substrate during film formation and/orthe heat treatment after the film formation contributes not only toimproving field-effect mobility but also to making the transistornormally off. In a transistor in which an oxide semiconductor film thatincludes In, Sn, and Zn as main components and is formed without heatinga substrate intentionally is used as a channel formation region, thethreshold voltage tends to be shifted negatively. However, when theoxide semiconductor film formed while heating the substrateintentionally is used, the problem of the negative shift of thethreshold voltage can be solved. That is, the threshold voltage isshifted so that the transistor becomes normally off; this tendency canbe confirmed by comparison between FIGS. 20A and 20B

Note that the threshold voltage can also be controlled by changing theratio of In, Sn, and Zn; when the composition ratio of In, Sn, and Zn is2:1:3, a normally-off transistor is expected to be formed. In addition,an oxide semiconductor film having high crystallinity can be obtained bysetting the composition ratio of a target as follows: In:Sn:Zn=2:1:3.

The temperature of the intentional heating of the substrate or thetemperature of the heat treatment is 150° C. or higher, preferably 200°C. or higher, further preferably 400° C. or higher. When film formationor heat treatment is performed at a high temperature, the transistor canbe normally off.

By intentionally heating the substrate during film formation and/or byperforming heat treatment after the film formation, the stabilityagainst a gate-bias stress can be increased. For example, when a gatebias is applied with an intensity of 2 MV/cm at 150° C. for one hour,drift of the threshold voltage can be less than ±1.5 V, preferably lessthan ±1.0 V.

A BT test was performed on the following two transistors: Sample 1 onwhich heat treatment was not performed after formation of an oxidesemiconductor film, and Sample 2 on which heat treatment at 650° C. wasperformed after formation of an oxide semiconductor film.

First, V_(g)−I_(d) characteristics of the transistors were measured at asubstrate temperature of 25° C. and V_(d) of 10 V. Note that V_(d)refers to drain voltage (a potential difference between a drain and asource). Then, the substrate temperature was set to 150° C. and V_(d)was set to 0.1 V. After that, 20 V of V_(g) was applied so that theintensity of an electric field applied to gate insulating layers 608 was2 MV/cm, and the condition was kept for one hour. Next, V_(g) was set to0 V. Then, V_(g)−I_(d) characteristics of the transistors were measuredat a substrate temperature of 25° C. and V_(d) of 10 V. This process iscalled a positive BT test.

In a similar manner, first, V_(g)−I_(d) characteristics of thetransistors were measured at a substrate temperature of 25° C. and V_(d)of 10 V. Then, the substrate temperature was set at 150° C. and V_(d)was set to 0.1 V. After that, −20 V of V_(g) was applied so that theintensity of an electric field applied to the gate insulating layers 608was −2 MV/cm, and the condition was kept for one hour. Next, V_(g) wasset to 0 V. Then, V_(g)−I_(d) characteristics of the transistors weremeasured at a substrate temperature of 25° C. and V_(d) of 10 V. Thisprocess is called a negative BT test.

FIGS. 21A and 21B show a result of the positive BT test of Sample 1 anda result of the negative BT test of Sample 1, respectively. FIGS. 22Aand 22B show a result of the positive BT test of Sample 2 and a resultof the negative BT test of Sample 2, respectively.

The amount of shift in the threshold voltage of Sample 1 due to thepositive BT test and that due to the negative BT test were 1.80 V and−0.42 V, respectively. The amount of shift in the threshold voltage ofSample 2 due to the positive BT test and that due to the negative BTtest were 0.79 V and 0.76 V, respectively. It is found that, in each ofSample 1 and Sample 2, the amount of shift in the threshold voltagebetween before and after the BT tests is small and the reliabilitythereof is high

The heat treatment can be performed in an oxygen atmosphere;alternatively, the heat treatment may be performed first in anatmosphere of nitrogen or an inert gas or under reduced pressure, andthen in an atmosphere including oxygen. Oxygen is supplied to the oxidesemiconductor after dehydration or dehydrogenation, whereby an effect ofthe heat treatment can be further increased. As a method for supplyingoxygen after dehydration or dehydrogenation, a method in which oxygenions are accelerated by an electric field and implanted into the oxidesemiconductor film may be employed.

A defect due to oxygen deficiency is easily caused in the oxidesemiconductor or at an interface between the oxide semiconductor and afilm in contact with the oxide semiconductor; however, when excessoxygen is included in the oxide semiconductor by the heat treatment,oxygen deficiency caused constantly can be compensated for with excessoxygen. The excess oxygen is oxygen existing mainly between lattices.When the concentration of excess oxygen is set to higher than or equalto 1×10¹⁶/cm³ and lower than or equal to 2×10²⁰/cm³, excess oxygen canbe included in the oxide semiconductor without causing crystaldistortion or the like.

When heat treatment is performed so that at least part of the oxidesemiconductor includes crystal, a more stable oxide semiconductor filmcan be obtained. For example, when an oxide semiconductor film which isformed by sputtering using a target having a composition ratio ofIn:Sn:Zn=1:1:1 without heating a substrate intentionally is analyzed byX-ray diffraction (XRD), a halo pattern is observed. The formed oxidesemiconductor film can be crystallized by being subjected to heattreatment. The temperature of the heat treatment can be set asappropriate; when the heat treatment is performed at 650° C., forexample, a clear diffraction peak can be observed in an X-raydiffraction analysis.

An XRD analysis of an In—Sn—Zn—O film was conducted. The XRD analysiswas conducted using an X-ray diffractometer D8 ADVANCE manufactured byBruker AXS, and measurement was performed by an out-of-plane method.

Sample A and Sample B were prepared and the XRD analysis was performedthereon. A method for manufacturing Sample A and Sample B will bedescribed below.

An In—Sn—Zn—O film with a thickness of 100 nm was formed over a quartzsubstrate that had been subjected to dehydrogenation treatment.

The In—Sn—Zn—O film was formed with a sputtering apparatus with a powerof 100 W (DC) in an oxygen atmosphere. An In—Sn—Zn—O target having anatomic ratio of In:Sn:Zn=1:1:1 was used as a target. Note that thesubstrate heating temperature in film formation was set at 200° C. Asample manufactured in this manner was used as Sample A.

Next, a sample manufactured by a method similar to that of Sample A wassubjected to heat treatment at 650° C. As the heat treatment, heattreatment in a nitrogen atmosphere was first performed for one hour andheat treatment in an oxygen atmosphere was further performed for onehour without lowering the temperature. A sample manufactured in thismanner was used as Sample B.

FIG. 25 shows XRD spectra of Sample A and Sample B. No peak derived fromcrystal was observed in Sample A, whereas peaks derived from crystalwere observed when 2θ was around 35 deg. and at 37 deg. to 38 deg. inSample B.

As described above, by intentionally heating a substrate duringdeposition of an oxide semiconductor including In, Sn, and Zn as maincomponents and/or by performing heat treatment after the deposition,characteristics of a transistor can be improved.

These substrate heating and heat treatment have an effect of preventinghydrogen and a hydroxyl group, which are unfavorable impurities for anoxide semiconductor, from being included in the film or an effect ofremoving hydrogen and a hydroxyl group from the film. That is, an oxidesemiconductor can be purified by removing hydrogen serving as a donorimpurity from the oxide semiconductor, whereby a normally-off transistorcan be obtained. The purification of an oxide semiconductor enables theoff-state current of the transistor to be 1 aA/μm or lower. Here, theunit of the off-state current is used to indicate current per micrometerof a channel width.

FIG. 26 shows a relation between the off-state current of a transistorand the inverse of substrate temperature (absolute temperature) atmeasurement. Here, for simplicity, the horizontal axis represents avalue (1000/T) obtained by multiplying an inverse of substratetemperature at measurement by 1000.

Specifically, as shown in FIG. 26, the off-state current can be 1 aA/μm(1×10⁻¹⁸ A/μm) or lower, 100 zA/μm (1×10⁻¹⁹ A/μm) or lower, and 1 zA/μm(1×10⁻²¹ A/μm) or lower when the substrate temperature is 125° C.(398.15 K), 85° C. (358.15 K), and room temperature (27° C., 300.15 K),respectively. Preferably, the off-state current can be 0.1 aA/μm(1×10⁻¹⁹ A/μm) or lower, 10 zA/μm (1×10⁻²⁰ A/μm) or lower, and 0.1 zA/μm(1×10⁻²² A/μm) or lower at a substrate temperature of 125° C., asubstrate temperature of 85° C., and room temperature, respectively.

Note that in order to prevent hydrogen and moisture from being includedin the oxide semiconductor film during formation thereof, it ispreferable to increase the purity of a sputtering gas by sufficientlysuppressing leakage from the outside of a deposition chamber anddegasification through an inner wall of the deposition chamber. Forexample, a gas with a dew point of −70° C. or lower is preferably usedas the sputtering gas in order to prevent moisture from being includedin the film. In addition, it is preferable to use a target which ispurified so as not to include impurities such as hydrogen and moisture.Although it is possible to remove moisture from a film of an oxidesemiconductor including In, Sn, and Zn as main components by heattreatment, a film which does not include moisture originally ispreferably formed because moisture is released from the oxidesemiconductor including In, Sn, and Zn as main components at a highertemperature than from an oxide semiconductor including In, Ga, and Zn asmain components.

The relation between the substrate temperature and electriccharacteristics of a transistor formed using Sample B, on which heattreatment at 650° C. was performed after formation of the oxidesemiconductor film, was evaluated.

The transistor used for the measurement has a channel length L of 3 μm,a channel width W of 10 μm, Lov of 0 μm, and dW of 0 μm. Note that V_(d)was set to 10 V. Note that the substrate temperature was −40° C., −25°C., 25° C., 75° C., 125° C., and 150° C. Here, in a transistor, thewidth of a portion where a gate electrode overlaps with one of a pair ofelectrodes is referred to as Lov, and the width of a portion of the pairof electrodes, which does not overlap with an oxide semiconductor film,is referred to as dW.

FIG. 23 shows the V_(g) dependence of I_(d) (a solid line) andfield-effect mobility (a dotted line). FIG. 24A shows a relation betweenthe substrate temperature and the threshold voltage, and FIG. 24B showsa relation between the substrate temperature and the field-effectmobility.

From FIG. 24A, it is found that the threshold voltage gets lower as thesubstrate temperature increases. Note that the threshold voltage isdecreased from 1.09 V to −0.23 V in the range from −40° C. to 150° C.

From FIG. 24B, it is found that the field-effect mobility gets lower asthe substrate temperature increases. Note that the field-effect mobilityis decreased from 36 cm²/Vs to 32 cm²/Vs in the range from −40° C. to150° C. Thus, it is found that variation in electric characteristics issmall in the above temperature range.

In a transistor in which such an oxide semiconductor including In, Sn,and Zn as main components is used as a channel formation region, afield-effect mobility of 30 cm²/Vsec or higher, preferably 40 cm²/Vsecor higher, further preferably 60 cm²/Vsec or higher can be obtained withthe off-state current maintained at 1 aA/μm or lower, which can achieveon-state current needed for an LSI. For example, in an FET where L/W is33 nm/40 nm, an on-state current of 12 μA or higher can flow when thegate voltage is 2.7 V and the drain voltage is 1.0 V. In addition,sufficient electric characteristics can be ensured in a temperaturerange needed for operation of a transistor. With such characteristics,an integrated circuit having a novel function can be realized withoutdecreasing the operation speed even when a transistor including an oxidesemiconductor is also provided in an integrated circuit formed using aSi semiconductor.

Example 1

In this example, an example of a transistor in which an In—Sn—Zn—O filmis used as an oxide semiconductor film will be described with referenceto FIGS. 27A and 27B.

FIGS. 27A and 27B are a top view and a cross-sectional view of acoplanar transistor having a top-gate top-contact structure. FIG. 27A isthe top view of the transistor. FIG. 27B illustrates cross section A-Balong dashed-dotted line A-B in FIG. 27A.

The transistor illustrated in FIG. 27B includes a substrate 1100; a baseinsulating film 1102 provided over the substrate 1100; a protectiveinsulating film 1104 provided in the periphery of the base insulatingfilm 1102; an oxide semiconductor film 1106 provided over the baseinsulating film 1102 and the protective insulating film 1104 andincluding a high-resistance region 1106 a and low-resistance regions1106 b; a gate insulating layer 1108 provided over the oxidesemiconductor film 1106; a gate electrode 1110 provided to overlap withthe oxide semiconductor film 1106 with the gate insulating layer 1108positioned therebetween; a sidewall insulating film 1112 provided incontact with a side surface of the gate electrode 1110; a pair ofelectrodes 1114 provided in contact with at least the low-resistanceregions 1106 b; an interlayer insulating film 1116 provided to cover atleast the oxide semiconductor film 1106, the gate electrode 1110, andthe pair of electrodes 1114; and a wiring 1118 provided to be connectedto at least one of the pair of electrodes 1114 through an opening formedin the interlayer insulating film 1116.

Although not illustrated, a protective film may be provided to cover theinterlayer insulating film 1116 and the wiring 1118. With the protectivefilm, a minute amount of leakage current generated by surface conductionof the interlayer insulating film 1116 can be reduced and thus theoff-state current of the transistor can be reduced.

Example 2

In this example, another example of a transistor in which an In—Sn—Zn—Ofilm is used as an oxide semiconductor film will be described.

FIGS. 28A and 28B are a top view and a cross-sectional view whichillustrate a structure of a transistor manufactured in this example.FIG. 28A is the top view of the transistor. FIG. 28B is across-sectional view along dashed-dotted line A-B in FIG. 28A.

The transistor illustrated in FIG. 28B includes a substrate 600; a baseinsulating film 602 provided over the substrate 600; an oxidesemiconductor film 606 provided over the base insulating film 602; apair of electrodes 614 in contact with the oxide semiconductor film 606;a gate insulating layer 608 provided over the oxide semiconductor film606 and the pair of electrodes 614; a gate electrode 610 provided tooverlap with the oxide semiconductor film 606 with the gate insulatinglayer 608 positioned therebetween; an interlayer insulating film 616provided to cover the gate insulating layer 608 and the gate electrode610; wirings 618 connected to the pair of electrodes 614 throughopenings formed in the interlayer insulating film 616; and a protectivefilm 620 provided to cover the interlayer insulating film 616 and thewirings 618.

As the substrate 600, a glass substrate can be used. As the baseinsulating film 602, a silicon oxide film can be used. As the oxidesemiconductor film 606, an In—Sn—Zn—O film can be used. As the pair ofelectrodes 614, a tungsten film can be used. As the gate insulatinglayer 608, a silicon oxide film can be used. The gate electrode 610 canhave a stacked structure of a tantalum nitride film and a tungsten film.The interlayer insulating film 616 can have a stacked structure of asilicon oxynitride film and a polyimide film. The wirings 618 can eachhave a stacked structure in which a titanium film, an aluminum film, anda titanium film are formed in this order. As the protective film 620, apolyimide film can be used.

Note that in the transistor having the structure illustrated in FIG.28A, the width of a portion where the gate electrode 610 overlaps withone of the pair of electrodes 614 is referred to as Lov. Similarly, thewidth of a portion of the pair of electrodes 614, which does not overlapwith the oxide semiconductor film 606, is referred to as dW.

This application is based on Japanese Patent Application serial no.2010-176982 filed with the Japan Patent Office on Aug. 6, 2010 andJapanese Patent Application serial no. 2011-108051 filed with the JapanPatent Office on May 13, 2011, the entire contents of which are herebyincorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a line; and amemory cell comprising a first transistor, a second transistor and acapacitor, wherein the first transistor is a p-channel transistor andcomprises a first gate electrode, a first source electrode, a firstdrain electrode, and a first channel formation region, wherein a firstinsulating layer covers the first transistor, wherein the firstinsulating layer is over the first channel formation region, wherein thesecond transistor comprises a second gate electrode, a second sourceelectrode, a second drain electrode, and a second channel formationregion over the first insulating layer, wherein a second insulatinglayer is over the second gate electrode, wherein the second insulatinglayer includes aluminum and oxygen, wherein the first gate electrode,one of the second source electrode and the second drain electrode, andone electrode of the capacitor are electrically connected to each otherto form a node where charge is held, wherein the line, one of the firstsource electrode and the first drain electrode, and the other of thesecond source electrode and the second drain electrode are electricallyconnected to each other, wherein the first channel formation regionincludes silicon, and wherein the second channel formation regionincludes an oxide semiconductor.
 2. The semiconductor device accordingto claim 1, wherein the second transistor is provided so as to overlapwith at least part of the first transistor.
 3. The semiconductor deviceaccording to claim 1, wherein an n-channel transistor is used as thesecond transistor.
 4. A semiconductor device comprising: a line; and amemory cell comprising a first transistor, a second transistor and acapacitor, wherein the first transistor is a p-channel transistor andcomprises a first gate electrode, a first source electrode, a firstdrain electrode, and a first channel formation region, wherein a firstinsulating layer covers the first transistor, wherein the firstinsulating layer is over the first channel formation region, wherein thesecond transistor comprises a second gate electrode, a second sourceelectrode, a second drain electrode, and a second channel formationregion over the first insulating layer, wherein a second insulatinglayer is over the second gate electrode, wherein the second insulatinglayer includes aluminum and oxygen, wherein the first gate electrode,one of the second source electrode and the second drain electrode, andone electrode of the capacitor are electrically connected to each otherto form a node where charge is held, wherein the line, one of the firstsource electrode and the first drain electrode, and the other of thesecond source electrode and the second drain electrode are electricallyconnected to each other, wherein the first channel formation regionincludes silicon, wherein the second channel formation region includesan oxide semiconductor, and wherein an off-state current per micrometerof a channel width of the second transistor at room temperature is lowerthan or equal to 100 zA.
 5. The semiconductor device according to claim4, wherein the second transistor is provided so as to overlap with atleast part of the first transistor.
 6. The semiconductor deviceaccording to claim 4, wherein an n-channel transistor is used as thesecond transistor.
 7. A semiconductor device comprising: a line; and amemory cell comprising a first transistor, a second transistor and acapacitor, wherein the first transistor is a p-channel transistor andcomprises a first gate electrode, a first source electrode, a firstdrain electrode, and a first channel formation region, wherein a firstinsulating layer covers the first transistor, wherein the firstinsulating layer is over the first channel formation region, wherein thesecond transistor comprises a second gate electrode, a second sourceelectrode, a second drain electrode, and a second channel formationregion over the first insulating layer, wherein a second insulatinglayer is over the second gate electrode, wherein the second insulatinglayer includes aluminum and oxygen, wherein the first gate electrode,one of the second source electrode and the second drain electrode, andone electrode of the capacitor are electrically connected to each otherto form a node where charge is held, wherein the line, one of the firstsource electrode and the first drain electrode, and the other of thesecond source electrode and the second drain electrode are electricallyconnected to each other, wherein the first channel formation regionincludes silicon, wherein the second channel formation region includesan oxide semiconductor, wherein the semiconductor device implements acircuit diagram, and wherein one of the first source electrode and thefirst drain electrode, and the other of the second source electrode andthe second drain electrode are directly connected to each other in thecircuit diagram.
 8. The semiconductor device according to claim 7,wherein the second transistor is provided so as to overlap with at leastpart of the first transistor.
 9. The semiconductor device according toclaim 7, wherein an n-channel transistor is used as the secondtransistor.